Necromancer refers to a broad category of mages that practice necromancy, which most famously makes and maintains the Undead, though it also interacts with life energy and souls. Necromancy is for example technically used in most Resurrection magic, or that which speaks to the dead in the River of Death. The forms that use the undead are under social taboo and often against the Law, whereas the forms that bring back the temporarily dead, especially when done by a Divine Magic Cleric, are much more accepted. The latter often makes use of diamonds, which has lead to the Diamond Crisis of Mogxa.
Necromancer also refers to a more specific class that uses Occult Magic magic and necromancy, and which makes use of a number of summoned and re-animated undead minions on the battlefield.
Liches are a type of monstrous necromancer whom have changed their own bodies to support immortality.