Alleged dead people who have nothing better to do with their afterlife than to frighten alive people and annoy house pets, especially black cats.

Without any kind of body to speak of, it is difficult to both illuminate and eliminate ghosts in most of the normal ways. Judicious use of talcum powder in the exact spot where your black cat is staring wide eyed with its fur pointing straight up, is a good way of temporarily revealing said spirit. At this point be quick with your prayer, and you will rid yourself of at least one of the immortal beasts. Repeat the process until you feel you have won.