Below is information complied by Ronan, on the ALFA Forums, on how spells work (in RP and PnP mechanics). For more information, see the sources at the bottom of this post.
The Magic of Faerun
Faerun is a land high in magic. Ancient mysteries, burried lore and forgotten power lay waiting for those bold enough to grab them. Yet to the common Faerunian, magic is a rare, wounderous thing. Few have the talent for it, and those who do are in high demand for their skills. Nearly all commoners have seen minor magics and glammers in their lives, though few own even the simplest of magic themselves. Well-off merchants or lesser nobles may own some minor forms of permanent magic, but few own the sorts of magic which the truely wealthy or successful adventurers do. This is because magic is both powerful and rare, and thus usually only comes into the possession of the bold or the very wealthy.
There are four main sources of magical items in Faerun. The first ist the distant past, ages of magic beyond anything known in the modern Realms. The second are the churches and religions across Faerun, who make magical items to further the aims of their deities. The third are simply individual wizards, whether traders or hermits. The fourth are the Red Wizards of Thay, a large organization of merchantile mages dedicated to a conquest of economy.
All magical effects in FR use the weave (except for shadow weave magic, which is very rare, and has to be learned bout IC in any case). This includes all spells, supernatural abilities, potions, etc. Even if a spell is cast by a diety, the weave is used, although other inate powers of the gods do not use it. In short, if it looks magical, it probably uses the weave. Even if magical effects are not powered by the weave but something else (such as the shadow weave or psionics) they are still magical in nature, and can be detected and dispelled by weave-spells.
Mystra, the goddess of magic, not only controls the weave, but she is the weave. Things that harm the weave harm her as well, as the weave is her body. Though she can cut anything (even deities) off from the weave, doing so is draining and distracting for her.
The weave, however, is not the source of magic itself. It is a gateway to raw magic, something which only gods can wield. The weave defines the way mortals can use magic and allows them to access it (think of a wall socket for accessing electricity). The symbols, gestures and words used by spell casters all touch the weave in certain ways to produce desired effects, and it can be thought of as a system that allow mortals to access raw magic.
The Art and the Power
Arcane magic, often called the Art, is any magic not granted by a deity. Used either intuitively (sorcerers, bards, etc) or from study (wizards), it is a colection of gestures, words, objects and thoughts that touch the weave. Arcane texts are written in a "language" that was discovered, not invented. As with divine magic, an arcane caster needs no understanding of why touching the weave does what it does, only how to touch it to produce the desired results. Casting an arcane spell often "feels" different for each arcane caster. Magic of Faerun gives examples of these sensations, but they are totally subjective to the caster.
The knowlege to cast divine magic, sometimes called the Power, originates from a deity. A divine spellcaster does not need any talent in touching the weave himself, as the power to access the weave is implanted directly into the mind of the caster by their mind by their deity (usually during prayer). The weave still powers the spell (although some may not even know this), but their connection to their deity is what allows them to draw on its power. Divine magic is usually more suitable when channeling positive of or negative energy (life/death spells). The Power also has much less complicated gestures involved, and of course is not subject to spell failure when casting in armor. Divine casters often feel sensations appropraite for their faith. Magic of Faerun gives examples of a cleric of Lathander feeling a warm heat on their backs, or a Kelemvorite cleric feeling the crunch of bones under his feet. Unlike arcane casters, these sensations remain the same between clerics of the same faith.
Powers that can grant Spells
Any deity (from demigod to greater) with a presence in FR can grant spells. Arch-fiends can also grant spells, usually in return for sacrifices and the like. If the "worshiper's" sacrifice quota is not met, he is often slain and drawn into the fiend's home plane.
The Lost Empires of Faerun book offers a 1st-level only feat called Servant of the Fallen which allows clerics of dead deities to be granted spells and raised from the dead.
Damage to the Weave
When the weave is damaged, magic goes awry. A dead magic zone is a hole in the weave, where it simply does not exist. No magic, supernatural powers or magic items can function in a zone of dead magic. Any creature with magical or supernatural powers (or an elf) becomes uneasy in a zone of dead magic, and could map out the boundaries of the zone by this feeling. A detect magic cantrip can easily show the caster the exact boundaries, however. A wish or miracle spell can repair the weave at a radius of 30 feet around the caster, but the spell of course must be cast outside of the dead magic zone.
Wild magic areas are those in which the weave is damage and frayed. It cannot be felt by magical creatures like a zone of dead magic, but can still be detected by a detect magic cantrip. Studying it with such a spell, the area appears to be magical at first. Then after a second round of studying, the caster can see it for a wild magic area. On the third round, the caster can make a DC 25 spellcraft check determine the boundaries of the area. The wild magic area literally makes all spells cast within it wild, and their effects can vary greatly. Supernatural powers and spells that were cast outside the area remain unchanged. A wish or miracle spell can repair a zone of wild magic the same as a zone of dead magic.
An anti-magic field (as per the spell) does not actually damage the weave. It simply stretches it, creating a temporary zone of dead magic. When the spell ends the area turns to normal. This is important in that clerics of Mystra could use the spell without violating their dogma and harming Mystra.
In theory, a wish or miracle spell could be used to create a zone of wild or dead magic just as easy as one is repaired.
Magic on other Planes
Mystra's Weave only covers the Prime Material plane of Aber-Toril. Weave spellcasters can cast on other planes, of course, but the effects can vary a bit from standard. Magic is inherint to the planes, and mortal spellcasters need no "medium" to access raw magic in such a settings. In effect there is a "weave" of some sort that casters use, but it is not related to Toril's weave or Mystra in any way but function. The laws of reality simply function differently on the planes. The Planescape setting covers how different planes effect spells, but most of it is rather intuitive. Fire spell damage is increased on planes of fire, evil spells are hampered on planes of good, etc. If you really want to get technical, there are specific types of spells that do not work on specific planes. Spells that use the ethereal plane, for example, only function on the Prime.
Specific Types of Magic
Simply put, this is magic centered around the elements of air, water earth and fire. Air elementalists are often from Halruaa, and fire elementalists are often Thayan (as Kossuth, the elemental god of fire, is popular there).
This is simply storing spells in gems, which are destroyed apon use. In this respect, it is similar to scrolls and potions.
Elven High Magic
This form of magic is only practiced by elves, and has (literally) shaped the Realms more than any other subtype of magic. The primary difference between it any ordinary magic, is that elven high magic is communal, and most often cast in groups. By drawing energy from huge groups (hundreds or more) of other elves (even non-casters and sometimes non-elven mages), feats of magic can be produced that could not otherwise be possible. The most obvious examples of this are Mythals, the Sundering, (which blew Faerun apart, creating Evermeet in the center of the Trackless Sea) and the curse placed on dark elves, turning them into Drow and driving them underground. Very few player-characters would know the later two events were caused by high magic, but many have heard of Mythals. Most often however, high magic is used to fix large-scale problems such as droughts, blights, and the like. It can be constructive as destructive, growing entire forests or walled defenses overnight.
High magic is extremely rare, and only taught to specific elves (usually at least 450 years of age) after decades of observation. It can only be cast by elves, as the casting of it involves the use of the elven abilitiy to commune with each other, and in the case of a high mage, even the weave and life around them. All high magic is of course powered by Mystra, but it also is controlled by the elven diety of magic, Corellon Larethian.
3rd edition rules on high magic make it out as weaker epic magic which is learned slightly more easily, but is otherwise no different from normal epic magic. This conflicts with other 3rd and 2ed edition canon on the subject, so it isn't clear what rules would govern its casting. Generally such magic would be inaccessible to player characters anyways, unless it was dug up from ancient ruins such as in Myth Drannor or the High Moor. For specific high magic rituals, see the Cormanthyr sourcebook, but Players Guide to Faerun has a few as well.
The Nether Scrolls
These two sets of 50 scrolls are possible the most powerfull artifact in the Forgotten Realms. While they have no direct magical powers, they provide a seemingly-limitless supply of arcane knowlege. After the discovery of the Nether scrolls, the arcane might of Netheril increased at least a hundred-fold. The Netherese found the scrolls amung the ruins of Aryvandaar, an ambitious and evil sun elven empire. These elves however, had not dared to use the power of the scrolls themselves.
The creation of the scrolls was started by the sarrukh (a creator race of reptiles). Their empire was vast, and they sought to gather all magical knowlege from its boarders into one location, in order to consolidate their arcane power. This effort spawned its own secret society, and lasted thousands of years after the fall of the empires of the Creator Races. It included magic from the batrachi and aearee (although not fey or humans, the two other Creator Races) as well. Eventually the Golden Skins of the World Serpent were created, which are now known as the Nether Scrolls.
The "scrolls" appear to be sheets of gold or platinum, with arcane runes floating about the surface. Each time a scroll is read, different knowlege is gleaned. When a reader gains suffecient understanding, re-reading the scrolls provides knew knowlege and information. There did not seem to be any limit to the amount of information one scroll could contain.
No destructive magics can affect the scrolls. They can be physically destroyed, but will always reform themselves eventually.
Each set of the scrolls were seperated into five groups of ten each:
Arcanus Fundare These scrolls dealt with the basics of spellcasting, and the different magical shcools.
Magicus Creare These provide information on the construction of magical items.
Maior Creare These scrolls provided information on the concstruction of golems, wards, and even intelligent and sentient wards. They also discussed anti-magic and dead magic phenomenon.
Planus Mechanicus These went into detail on the planes, their magic, and how to create pocket planes.
Ars Factum These were the most difficult scrolls to understand, and detailed the construction of artifact-level magic items.
A full set of 50 scrolls were stolen by the elves of Cormanthyr. They all currently reside in the Windsong Tower in the ruins of Myth Drannor. For more information on the tower, see the Cormanthyr sourcebook. For more information on the scrolls, see the Netheril: Empire of Magic sourcebook. The Lost Empires of Faerun sourcebook tells of the resting places of various numbers of the other set of 50 scrolls. The Netherese never found the location of the scrolls, so this is not something PCs would find out about.
Famous Magical Creations
These are probably the most famous creations of elven high magic, although many mythals were created from wizardry magic (such as Myth Drannor's and Silverymoon's). They create huge spheres of magical energy which protect entire cities. They can regulate temperature, prevent certain types creatures from entering, empower or block certain spells, and many other things (such as flying at will, or even stopping inhabitants from aging). Over the ages (and the death of Mystryl and later Mystra), many mythals have become corrupted, causing random and sometimes harmfull magical effects within their area. The most well-known un-corrupted mythals on Faerun are in Silvermoon and Evereska.
Historians consider "true" mythals as those created by elven high magic. They are generally more powerfull, cover wider areas and last longer than other mythals, but otherwise function the same. Other "mythals" may be nothing more than a collection of smaller wards, and may be nearly functionally equal to a "true" mythal. For more information on Myth Drannor's mythal and high magic in general, see Cormanthyr: Empire of the Elves. For a list of true Mythals and 3rd edition rules on them, see Lost Empires of Faerun.
Mythallars: These ancient Netherese devices looked like huge, glowing crystal balls, 150 feet in diameter. Their function was to tap into the weave and allowed easier access to raw, unfiltered magic.
This seemingly-simple effect allowed many wonderous creations. Magical items could draw power from the mythallar, and be constructed much cheaper and easier. Such quasi-magical items would not function outside the mythallar. The magics that allowed Netherese enclaves to fly were powered by mythallars, and multiple, overlapping mythallars were needed to power many larger enclaves (each had a radius of one mile). Currently, the only well-known active mythallars in Faerun are on the floating city of Shade. For more information on Mythallars and Netherese magic, see Netheril: Empire of Magic.
Most spells are rather self-explanitory, and are listed on that d20srd links provided above. However, due to the lack of a few key spells in NWN, some things should be explained: A spellcaster cannot normally "sense" a spell or magical effect. If he can see a particular effect of the spell, he can make a spellcraft roll to identify it with a DC of 20 + the spell's level. If the spell has no noticable effect, a detect magic cantrip or similar must be used. If he sees or hears some part of the spell being cast, a spellcraft roll of 15 + the spell's level is needed to identify it. For more information, see the link on the spellcraft skill.
Detect magic, divination
http://www.d20srd.org/srd/spells/detectMagic.htm This is a cantrip all spellcasters get access to, and lasts as long as the caster concentrates, or up to one minute per level (I usually simulate it by casting resistance on an item in my inventory). Detect magic allows one to see all auras within 60 feet of the caster, and determine the power of each aura. The caster can make a spellcraft check of DC 15 + the spell's level to discern the school of magic of each aura. Note that magic auras will linger a bit after a spell has been cast, depending on the strength of the spell or effect. Arcane sight and greater arcane sight are improved versions of detect magic, and are only available to higher-level wizards and sorcerers.
Read magic, divination
http://www.d20srd.org/srd/spells/readMagic.htm Magic is writen in (of course) arcane symbols. These symbols were not invented, but discovered. However, each mage writes them in his own way, and one mage cannot always understand the writings of another (arcane texts can be thought of as mathematical formulas written by different mathmeticians). In order for someone to understand an arcane text, they need to make a spellcraft check of DC 20 + spell level (repeated as often as once a day), but the use of a read magic cantrip (something which all wizards can prepare without needing to reference a spellbook) can negate the need for this check. Note that even with read magic, a caster is not guarenteed to identify some symbol-based magical traps (see the link to the spellcraft skill, above).
~~A scrying spell creates an invisible magical sensor that sends you information. Unless noted otherwise, the sensor has the same powers of sensory acuity that you possess. This level of acuity includes any spells or effects that target you [such as darkvision, see invisible, etc], but not spells or effects that emanate from you [such as detect magic]. However, the sensor is treated as a separate, independent sensory organ of yours, and thus it functions normally even if you have been blinded, deafened, or otherwise suffered sensory impairment. Any creature with an Intelligence score of 12 or higher can notice the sensor by making a DC 20 Intelligence check. The sensor can be dispelled as if it were an active spell. Lead sheeting or magical protection blocks a scrying spell, and you sense that the spell is so blocked [common magical defenses to scrying include spells of nondetection and mind blank] ~~
Note that a creature only gets a will saving throw to disbelieve an illusion if it interacts with the illusion or studies it carefully. If the creature has solid proof (such as seeing his sword pass through an illusionary goblin) the illusion is not real, no saving throw is needed and the illusion is disbelieved. If a creature disbelieves the illusion, he can see its outline but is also able to see through it as if it was transparent.
Most PnP spells a PC in ALFA will use are available here: http://www.d20srd.org/indexes/spellLists.htm There are some FR-specific spells in the FRCS and other sources, and many other spells not specific to FR in books like the Complete Arcane and Divine. Of course, nothing says NPC spellcasters can't have their own spells different from those listed in the rules (such as a nigh-undetectable scying spell, or defenses against plot-ruining divinations). In the case of wizards, it takes them much time and gold to develope their own spells. Clerics could certainly have an unusual spell granted to them by their deity.
Psionics and the Weave
Although there are not Psionics in NWN, psionic powers of NPCs may be used by DMs. Psionics are quite different from magic, and the Player's Guide to Faerun says a few things about psionics: -As a whole, magic has had a far greater impact on the history of Faerun. As a result, most think Psionics are only wielded by specific beings, such as mind-flayers. -Psionics tap into the power of psion himself, not the weave or shadow weave. Shar and Mystra have no power over psionics. -Psionic effects are still magical in nature, and can be detected, dispelled and warded against by spells such as detect magic, dispell magic antimagic shell, and others.
Schools of Magic
Abjurations are protective spells. They create physical or magical barriers, negate magical or physical abilities, harm trespassers, or even banish the subject of the spell to another plane of existence.
Each conjuration spell belongs to one of five subschools. Conjurations bring manifestations of objects, creatures, or some form of energy to you (the summoning subschool), actually transport creatures from another plane of existence to your plane (calling), heal (healing), transport creatures or objects over great distances (teleportation), or create objects or effects on the spot (creation). Creatures you conjure usually, but not always, obey your commands.
Divination spells enable you to learn secrets long forgotten, to predict the future, to find hidden things, and to foil deceptive spells.
Enchantment spells affect the minds of others, influencing or controlling their behavior.
Evocation spells manipulate energy or tap an unseen source of power to produce a desired end. In effect, they create something out of nothing. Many of these spells produce spectacular effects, and evocation spells can deal large amounts of damage.
Illusion spells deceive the senses or minds of others. They cause people to see things that are not there, not see things that are there, hear phantom noises, or remember things that never happened.
Necromancy spells manipulate the power of death, unlife, and the life force. Spells involving undead creatures make up a large part of this school.
Transmutation spells change the properties of some creature, thing, or condition.
To many, it would seem a powerful magic user is nigh-unstoppable by ordinary defenses. He could scry on you from anywhere, teleport across the continent in the blink of an eye, or cause a castle's walls to dissolve with but a word. While this is sometimes true, there are both magical and mundane defenses against many spells, and only a fool would design a stronghold without magical defenses in mind. In practice it is rare to be able to organize enough casters of great power to raze a stronghold on their own. It is often far easier and often cheaper to get scores of soildiers to build things such as trebuchets to accomplish the same task.
Scrying is blocked by even a thin sheet of lead positioned around the target, and it is not uncommon for important rooms in wealthier houses or strongholds to be shielded in such a way. Gorgon's blood mixed in with construction materials serves the same purpose for spells which utilize other planes to travel, such as all conjuration spells of the teleportation subschool (teleportation, tree stride, word of recall, etc), and ethereal jaunt. Volo has this to say on the use of such things, [i]~~To be effective, the gorgon’s blood must be in a solution of one drop to a pint of water or stronger and must be applied so that no area of the external walls larger than a large man’s head is untouched by it. Xorn or medusa blood can be used instead, but it must be applied in the following complex formula: three drops of xorn blood or four drops of medusa blood and two drops of unholy water per pint of water. Needless to say, the second formula is not used within upon buildings belonging to or used by good or (most) neutral faiths.
[Teleportation] can be prevented by magical items such as weirdstones1 or by the presence of sufficient Underdark radiation (strange emanations from certain rocks in which the metal arandur is found). 2 These radiations fade swiftly if the rock containing them is exposed to sunlight, but if taken to the surface on moonless nights or cloaked in magical darkness, the ore can be used as a rubble filler within double walls to foil teleporters. Be warned that certain preservative spells not known to me, wondrous web spells, and magics that melt the rubble into a flowing, briefly molten mass must be used to make the protection of the radiations both continuous and long-lasting. Even with such precautions, use of Underdark ores is notorious for leaving as the turtle soup fanciers of Neverwinter say, “gaps in the shell,” so that teleportation is difficult and its destinations restricted, but complete prohibition rarely gained.~~[/i]
A spell of forbiddance or dimensional anchor attached to a spell of hallow can shield an area from dimensional travel and teleportations as well. Gates and the like can be shielded with elemental resistance spells, making them less likely to be damaged by spells such as fireballs. All but the most powerful of casters would likely find themselves slain before they were able to burrow through or destroy a keep's walls, as multiple castings of spells such as disintegrate or earthquake would be needed. Flying over the walls is an option, but that leaves one vunerable to the scores of archers and balistae which most fortifications employ.
Sources: Player's Handbook Forgotten Realms Campaign Setting Magic of Faerun Spider Jones Ballonger Candlekeep forums Serpent Kingdoms Volo's Guide to All Things Magical