The Weave
Mortals cannot directly shape raw magic. Instead, most who wield magic make use of the Weave. The Weave is the manifestation of raw magic, a kind of interface between the will of a spellcaster and the stuff of raw magic. Without the Weave, raw magic is locked away and inaccessible - an archmage can't light a candle in a dead magic zone. But, surrounded by the Weave, a spellcaster can shape lightning to blast her foes, transport herself hundreds of miles in the blink of an eye, even reverse death itself. All spells, magic items, spell-like abilities, and even supernatural abilities such as a ghost's ability to walk through walls, depend on the Weave and call upon it in different ways.
The exact nature of the Weave is elusive because it is many things simultaneously. The Weave is the body of Mystra, the goddess of magic. Mystra has dominion over magic worked throughout Toril, but she cannot shut off the flow of magic altogether without ceasing to exist herself. The Weave is the conduit spellcasters use to channel magical energy for their spells, both arcane and divine. Finally, the Weave is the fabric of esoteric rules and formulas that comprises the Art (arcane spellcasting) and the Power (divine spellcasting) Everything from the texts of arcane spellbooks to the individual components of spells is part of the Weave. Magic not only flows from source to spellcaster through the Weave, the Weave gives spellcasters the tools they need to shape magic to their purposes.
The Schools
Schools of magic, also known as schools of philosophy, are categories into which spells were organized by function. Spells were created by wizards with these schools in mind.
Spellcaster’s often chose to specialize in spells from a certain school; they focused more effort into these spells than any other, but at the expense of all spells from one or more other schools. These schools of magic had been in existence for ages and their origins were mostly unknown.
The major schools of magic are:
Abjuration: A collection of spells of a protective nature.
Transmutation: Formerly also known as alteration, these spells can transform the nature of the physical world or objects in it.
Conjuration: This group of spells creates or transports people, energy or objects.
Divination: This school has fewer spells than others, but allows the caster to see things that they normally would not be able to.
Enchantment: An area of arcane study that specialized in manipulating the minds of others.
Evocation/Invocation: Although spells of this school seemingly created effects out of nothing, they drew raw power from out of the Weave.
Illusion: This school was almost a secret society prior to the Time of Troubles, even possessing their own language. Their spells were those that fooled the senses.
Necromancy: A dichotomous school that wielded positive energy into healing spells, and negative energy to effect both the dead and undead.
Universal: A small number of spells were not associated with any school but universally available, even to specialists.
