Magic

Magic is the ability possessed by some individuals to manipulate the ambient energies of the world to produce desired results. In Realms, magic is commonly referred to as "the Art".

The goddess Mystra controls the Weave which is what powers the magical energies of Abeir-Toril. The goddess Shar has control of the Shadow Weave which flows in-between the normal weave and enables the use of Shadow Magic.

The Art
Any magic that doesn't originate from a deity is defined as arcane magic. (Note, while all magic is accessed through the Weave, which is maintained by a deity, this does not make all magic divine magic.). The use of arcane magic is refered to as the Art, in day to day speech, and a vide variety of people (and character classes) are able to practice the Art, to a smaller or larger extent, though the way in which the access the Weave can differ dramatically. Most wizards spend long years researching their art, gathering spells to their personal book, and each day they can only memorise a small fraction of these, the memory of the spell being wiped from their mind as it is cast. The wizard has to re-study the spell before he or she can cast it again, unless more than one casting of the spell in question was prepared. Sorcerers, also known as innanoths (for their innate mastery of the weave) are not required to research spells, they tap the Weave in a more direct manner, but because of this the selection of spells available is all the more limmited to a Sorcerer than a wizard. Bards, Assassins and many other prestige classes access the weave in some way as well to use certain magical abilities, practicioners of the Art all, wether or not they would acknowledge such kinship with each other.

The Power
Magic that originates from a spell-granting deity, usually through prayer, is divine in nature and is called the Power by the common folk. Clerics, druids, paladins, rangers and many prestige classes all derive their spell and spell-like abilities from a deity. A practicioner of the Power has no affinity with the Art, as their spells are planted in their minds directly by their patron deity, and they do not tap the Weave. Casting divine spells is more like an exclamation of faith that brings about a sensation appropriate to the patron deity who the faith was devoted.