Dead folks who, whether due to dark magic, mad science, lack of a proper funeral or burial, some old problems they still haven't solved, a desire to get revenge on their killers or enemies, or just plain stubborn bloody-mindedness, do not rest in peace. Unable to move on to the afterlife, they still roam around in the mortal world, (poorly) trying to behave as if they're still alive. One way undead vary is the nature of their mind, soul, and spirit. In some cases, including liches, most ghosts.