Magic is the system whereby a person adept within its arts can manipulate the world in various was, using reagents and various rituals. From various rituals to summon monsters, to the ability to conjure matter out of nothing.
Various people can use magic with various amounts of skill. Many become mages at the Collages of Magic. Some forms of magic are barred, such as necromancy, but most are allowed within civilized culture.
There are four major schools of magic: Conjuring, Necromancy, Elemental, and Healing.