An incubus is a male demon, and a succubus is a female demon. They both are associated with sexual intercourse, and dreams.
Non-standard Christian and Jewish theology has all Incubi and succubi as descendants of Lilith, the first wife of Adam, and quite possibly of Adam as well. They are supposed to come to people in the night and have intercourse with them. This idea, I think, was incorporated by Hebrews from the Babylonians, who had a multitude of demons for every ill in the world. Christians use these demons to explain wet dreams, which coincidentally is a natural thing. The incubi and succubi take the form of the person that is most desired. Supposedly they suck the life from you little by little, and tempt your souls with sexual bliss. Open sexuality according to Christians must be the idea of the Devil. Well, I think not, fanatics. these "demons" were and are used by the various churches to maintain control of the sheep who so blindly follow the wolf, uh, I mean the shepherd.