Ghosts (often referred to as spirits), are noncorporeal, astral beings. With a few exceptions, they have translucent greenish bodies that can be interacted with and despite being metaphysical, they can take and deal physical damage. They don't leave a corpse behind when killed.
Ghosts can be friendly NPCs, neutral or hostile to the player, depending on their type.
For a list of Ghosts in the game, see: Category:Ghosts