Many of the creatures called "Undead" by human beings are nothing more than inert corpses, animated with the appearance of life by a form of sorcerous power. Others are effectively bestial creatures: Corpses of once-sentient beings acting as hosts to animating parasites with nothing resembling human sentience or intelligence at all. These types of creatures are normally known as Walking Dead and Ghouls, respectively, when the precise nature of the spell or parasite is not known. There do exist some sentient beings however who by the work of extraordinary sorcerous power -- sometimes but not always their own -- extend their existence in the world beyond the natural lives of their bodies, sometimes preserving their corpses to serve as the vessels for their sentient spirits, sometimes generating artificial bodies for themselves, and sometimes existing as entirely non-corporeal beings. In theory there is no reason to suppose that undead beings would be any more "good" or "evil" than living ones, but in practice the choice to become undead (when it is voluntary) is unlikely to be compatible with -- nor is the fact of existing in an Undead form for any length of time conducive to -- what most human beings would regard as sanity.
The sorcerous power necessary for the transition from living to undead being is such that nearly every case is unique; nevertheless, some commonalities have been inferred, such as the difficulty of maintaining undead existence in direct sunlight or the presence of large numbers of people. Just how universal these are however, and the possible nature of others, remains unknown.
Several broad categories of undead existence have been used to distinguish between them on the basis of their ties to their once-living bodies (or to any body at all). The best-known names for the best-known categories are:
Liches: Undead spirits still bound to their once-living bodies. This broad category includes beings cursed to live beyond death in decaying bodies (sometimes preserved by artificial means, as in the case of Mummies) and those whose bodies are preserved, renewed, or both by sorcerous means.
Vampires: Undead spirits bound to a version of their once-living bodies that are capable of sustaining themselves by various physical means (most commonly involving the drinking of blood to restore lost fluids, tissues, and energy).
Revenant Spirits: Undead spirits no longer tied to their actual once-living bodies that nevertheless interact with the world by embodying themselves (usually repeatedly, temporarily, and by sorcerous means).
Ghosts: Undead spirits no longer bound to their once-living bodies in any way, retaining their undead existence completely independent of any corporeal form they may seem to assume -- if any.
