The Weave

The Weave is a way through which raw magic is accessed, tapped into and used by casters of magic. 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 the exception of dead-magic zones. The Weave coexists with The Shadow Weave.
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. Many see it as a "fabric" on which magic is "drawn", and damaging the fabric causes magic to go awry. Preparing a spell is a means by which a small amount of the Weave can be "contained" and transported for a specific effect, while the casting of a spell is equivalent to telling the Weave to rearrange itself to create the effect.
Importantly, the Weave is not magic itself, but rather a force by which its flows can be channeled, and a means by which magic can be understood and harnessed. Other systems for calling upon magic have always existed within the Realms, but the Weave is by far the most commonly used, thoroughly refined, and best understood.