Weave

The Weave, controlled by Mystryl or one of her successors, was a way through which raw magic was accessed, tapped into and used by by casters of magic. With the death of Mystra in 1385 DR, the Weave collapsed. 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 or existence, with exception of dead-magic zones. The Weave coexisted with the Shadow Weave, and with the death of Mystra, Shar was unable to maintain the Shadow Weave and it collapsed as well.

Nature of the 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. It can be thought of as a "fabric" on which magic is "drawn", and damaging the fabric causes magic to go awry. There occurs some deliberate falsification of the nature of the Weave in textbooks, as an attempt to limit the knowledge associated with spellcasting.

Casting a spell is equivalent to telling the Weave to rearrange itself to create an effect.

Special powers
The Weave is able to grant extra powers to certain individuals. Silver fire is an ability granted by Mystra only to the Chosen of Mystra and enables its user to envelop itself in silvery white flames that bestow many potent magical powers. Spellfire is a rare talent that allows its user to manipulate the Weave in a variety of powerful ways.

Spellplague
Because Mystra (and formerly Mystryl) was inextricably bound to the Weave (one cannot exist without the other), when Mystra was assassinated by Cyric and Shar on 29 Tarsakh 1385 DR, the Weave collapsed and initiated the Spellplague. Henceforth it presented itself as scraps of burning crystal tumbling through an endless void.

In 1479 DR during the events of Elminster Enraged, it is revealed that Mystra foresaw her own fall and let it happen because the Weave was growing more and more unstable, and needed to be renewed as part of a cycle fated to repeat again and again.

Anomalies
Places such as dead-magic zones or wild magiczones are places where the Weave has been damaged or is non-existent.

Wild magic zones
In wild magic zones, a spell can backfire upon its caster, target the wrong location, be dramatically increased in power, or many other things. A wild magic zone and its parameters can be detected using a detect magic spell. A caster can temporarily fix a wild magic zone with any dispel or permanently fix it using a wish spell.

Cosmic disasters like the Time of Troubles or the Fall of Netheril caused the emergence of severe wild magic zones after the Weave was heavily damaged. The mythals of the elves can also create localized zones of wild magic, but these areas are under the control of the elves and they can define the parameters under which the mythal operates, such as which spells will not work and which are enhanced in some way.

Dead magic zones
Dead magic zones are places where the Weave is no longer in existence. Both it and its boundaries can be detected in the same way a wild magic zone can be detected, but from outside the zone. However, unlike wild magic zones, most casters innately know when they've stepped into a dead magic zone. A dead magic zone can only be fixed with a wish spell or silver fire.