Being undead is a condition under a formerly living organism ceases having the needs he/she had when living, but continues being able to perform most if not all living functions, despite being clinically dead. In other words, the condition of being a walking corpse.
Sometimes, we distinguish between two types of undead, which is by the manner in which the formerly living being becomes undead.
The more common type of undead is a corpse made living by act of Necromancy - turning a skeleton into a moving-fighting undead skeleton, or a decaying corpse into a zombie. These types of undead are not sapient and more often than not act as puppets of their creators or aimlessly roam the wild until eventually killed - the one and only exception to this are the Liches, who are created in a special ritual that involves the complete burning of one's full body alive until nothing but the skeleton remains, kept alive by the dark powers of the person performing the ritual on himself/herself.
The other well-known type of undead is when a formerly living organism for all purposes dead, but is kept alive (undead) by a certain disease, namely Vampirism or Lycantrophy. Just like Liches, these undead creatures are always sapient, but unlike Liches and other types of undead, they keep their mortal bodies fully intact, without any visible signs of their condition (other than vampires having unnatural eye colours and pale skin). It is also worth mentioning that Vampires and Lycantrophes are capable of sexual reproduction just like mortals, and when an underage person receives one of the two aforementioned diseases, he/she will still grow up, but stop ageing after having reached maturity. These types of undead are also well-known for still having another semblance of being mortal - the need for a substance: vampires and lycantrophes require blood to fuel their bodies with energy.