Undead are the animated corpses of once-living beings. Most are reanimated through necromancy or are the direct creation of demons. Though their methods of creation and their physical properties may differ, all undead are considered unnatural and unholy, and are rapidly burned by sunlight and holy magic.
Undead usually vary according to how they were created. Necromancers may summon undead slaves without the use of a captured soul, creating the slow-moving shambling dead, or they may bind a living soul to a dead body to create a more powerful revenant. Some undead occur "naturally" as inhabitants of the spirit realm - lingering souls who refuse to pass into the afterlife - attempt to contact their loved ones. These are called ghosts. Some ghosts whose willpower and hatred are strong enough to break the barrier into the mortal realm so they may cause direct harm to mortals.
In some cases, powerful necromancers have sought undeath as a way to achieve a sort immortality in this world. They bind their souls to physical anchors to prevent them from passing into the afterlife, thus becoming undead liches. The mysterious creatures known as vampires were created when the ancient race of bat-shifters sought the help of demons in order to retain their powers. They are now a type of undead who breed by infection, spreading their 'gift' to chosen victims.
More information about each type of undead can be found through the links above, or on the other pages in Category:Undead.