Undeath

Undeath: A state of existence beyond mortal life, this can take as many forms as there are types of undead creatures. The basic principles are divided into the active and passive undeath, the passive being mindless skeletons/zombies re-animated through magic or force of will. The active variety however can occur through several processes, here's a short list:

willforce: enspirited zombies/skeletons/ghosts who did not wish to leave the mortal realm after dying.

voodoo: mindless zombies controlled by a shaman/witchdoctor.

infection: Vampires/werewolves(not exactly undead as they are alive)/undead mutants(only in certain books/games such a Resident Evil).

ritual: Mummies,Liches and Demi-Liches.

The common weaknesses of undead include:

Sunlight

Religious objects/symbols/fluids (skeptically added)

Salt

Garlic

Silver

Fire

Purity of heart

A stake through their heart
However, undead are in some cases nigh impossible to kill due to regenerative abilities/discorpereal strength/resurrective backup or any other such thing.