Ghost

Ghosts are cursed beings whose mortal form has met their end, but whose consciousness remains in the mortal plane due to magic.

Specter
Specters are your average, run-of-the-mill ghostly apparition. They cannot leave the area they have been cursed to, but have some control over the environment itself. They retain very little of their physical appearance, and appear mostly as mist.

Phantom
Phantoms are a rarer breed of ghost. Though still locked to one area (although a generally larger spot of land), they can manipulate their environment mostly to their own will. They maintain much of their physical appearance, though it can appear changed and damaged.

Revenant
Revenants are generally considered a myth. They are a more powerful version of a phantom, key differences being that they cursed themselves on purpose to retain their form after death, and have vast control over their environment. Some even say that the particularly powerful revenants are not constrained to one location, and may roam free. They should not be confused with lichs, however: a lich is able to hold onto its actual physical body, whereas a revenant's form is an illusion. However, a revenant has more control over its own actions, while a lich's will is subject to easy corruption, as they are slowly taken over by the magic that binds them to life.