Mystra is the Goddess of Magic in the Forgotten Realms. Forgotten Realm is a collection of roleplaying/adventure books published by TSR -- the same people that brought you Dungeons and Dragons, and Dragonlance. She is the goddess of the power of magic itself, not the goddess of spellcasters, although many spellcasters (Elminster is an example) number themselves among her followers. To her alone is given the power to see through the eyes of the other gods and goddesses of the Forgotten Realms, that she may see how their strength of purpose, colors, even their perception of reality. The Lady of Mysteries is the greatest of the greater powers of the Realms.
During the fall of the Gods in Forgotten Realms, Mystra perishes and Midnight takes up her place. This is detailed in the Avatar trilogy and somewhat also in the Fall of the Avatars trilogy.