Non living monsters and, in some sytems, players. Examples: Skeletons, ghosts, vampires, zombies, mummys, etc.
corpses animated with the power of unlife, whether grossly or subtly. Undead beings range from the crude skeletons and zombies to the sophisticated vampires and liches.
Undead are fallen warriors who have been brought back to life to fight for the horde under, Sylvanas Windrunner. For more info see: WoW Community Site: Undead
Monsters brought back from death to a cruel mockery of life. Some of them are mindless drones, others are conscious and cunning, but all of them lack a soul. For example, zombies, skeletons and vampires, are undead.