The Undead, or risen dead are a class of monster, or unnatural horror. While the term is often used to refer to Zombies, zombies are merely one type of Undead (albeit the only type confirmed to exist in some form or fashion, and the only type this wiki is concerned with).
An Undead specimen is one that was once alive, died, and then rose again in contradiction to mankind's understanding of life and death.
The vast majority of the time this happens in literature, the detachment the subject has had from life brings with it malicious motives. Fear of death is a nearly universal sentiment in mankind, and literary personification of this sentiment is as old as myth or storytelling itself.