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 casters of magic. The Weave was the way in which magic presented itself to beings for their use, and it flowed throughout the world, touching almost every corner of existence, with exception of dead-magic zones. The Weave coexisted with the Shadow Weave.

The Weave did not existed on Abeir, making casting magic in that world more difficult than in Toril.

Nature of the Weave
The Weave was 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 existed around the planes. Many saw it as a "fabric" on which magic was "drawn", and damaging the fabric caused magic to go awry. Some textbooks deliberately falsified the nature of the Weave, as an attempt to limit the knowledge associated with spellcasting.

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

Special powers
The Weave was able to grant extra powers to certain individuals. Silver fire was an ability granted by Mystra only to the Chosen of Mystra and enabled its users to envelop themselves in silvery white flames that bestowed many potent magical powers. Spellfire was a rare talent that allowed its user to manipulate the Weave in a variety of unique and 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 Tarsakh 29 1385 DR, the Weave collapsed and initiated the Spellplague. Henceforth it presented itself as scraps of burning crystal tumbling through an endless void. With the death of Mystra, Shar was unable to maintain the Shadow Weave and it collapsed as well.

In 1479 DR it was 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. In 1487 DR the Weave was almost repaired to its former state.

Anomalies
Places such as dead-magic zones or wild magiczones were places where the Weave had been damaged or was non-existent.

Wild magic zones
In wild magic zones, a spell could backfire upon its caster, target the wrong location, be dramatically increased in power, or many other things. A wild magic zone and its parameters could be detected using a detect magic spell. A caster could 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 could also create localized zones of wild magic, but these areas were under the control of the elves and they defined the parameters under which the mythal operated, such as which spells would not work and which were enhanced in some way.

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