The Weave is the means by which raw magic is tapped into and used by spellcasters. The Weave is the way in which magic presents itself to beings for their use, and it flows throughout the world, touching almost every corner of existence, with exception of dead magic zones.
The Weave is considered many things, including Mystra's body, the source of magic, all the studies of casters, arcane and divine alike, and the many energies and forces that exist around the planes. Casting a spell is merely telling the Weave to rearrange itself to create an effect.
The Weave is regulated and maintained by the goddess Mystra, who embodies the heart of magic in her home plane of Dweomerheart.
Some places have anomalies in the Weave, in wild magic zones or dead magic zones. A side effect of the Time of Troubles and other extraordinary events, these wild magic zones exhibit erratic and unstable effects of magic, while dead magic zones have no function of magic available.
The dark goddess Shar created a rival force called the Shadow Weave, which is the inverse of the Weave and functions separately. As it is separate, it can still function normally in wild or dead magic zones. However, if the two forces directly collide at an area, it creates a tear in spacetime, often causing "holes" leading to other planes.