Ghosts are ethereal beings that return to the world of the living for one reason or another, often to exact revenge or simply because they weren't ready to rest yet.
There exist several types of ghosts, as seen below.
Spectre: Usually peaceful spirits that simply find it hard to carry over onto the next world. Spirits of animals that appear in a ghastly form are usually spectres.
Phantom: Ghosts that haven't fully carried over to the world of the living, partially remaining in the ethereal world. They are rarely seen in physical form, more often appearing as a gust of wind or recognised as a silent voice.
Wraith: Vengeful spirits that return to the world of the living only to exact revenge. Once their business is done, they leave the same way they came, never to return.
Bodach: Ghosts that appear entirely human, but are infact ethereal beings. People that have encountered bodachs describe the experience as 'hypnotic'.
Mirage: Spirits that have lost their consciousness, and now continuously act out past events in the material world as if they were real.
Eidolon: Rested spirits that are forcibly summoned to the world of the living through some kind of ritual. They may appear in their mortal form, but with the essence of an ethereal creature, or alternatively they may take over a dead mortal's body and become material creatures again. This kind of ritual is very difficult to perform, and requires ancient knowledge that few people possess.
Haunt: Evil spirits that feed off the fear of mortal creatures. They always appear at night.