A necromancer is a spellcaster who ressurects the dead to do his/her bidding. Most are mages, exiled from the Hand Brotherhood for practicing the forbidden art. These mages established an order of their own in a castle somewhere north of The City. The castle is never seen as it was scrapped with Thief II Gold.
A professional necromancer's primary aim is to learn the secrets of life and death written within the Book of Ash, an ancient tome of immense power. Although many have studied the book, most or all were driven insane by the process.
Necromancy is the act of talking to the dead, summing spirits to give knowledge via divination. However, it is mostly used as a term to mean people that can summon up the dead, like zombies in modern culture. In Thief, both seem to be the case as many have been seen to gather the knowledge from the dead, while others have been seen to raise the dead, and both are given the same term of 'necromancer'.