Demons are a part of many cultures theologies, though they are not always called so. largely they are considered as part of monotheistic religions, like Christianity, but are also seen in wide variety in Asiatic cultures.
Creature Cheat Sheet:
Type: Magical, Spirit, possible Beast
Habitat: Depends on the Demon and cultural basis
Possible Origin: Greece? Japan?
Best Seen: Supernatural
Representations of demons can depend on the writer. Some can have special abilities and vary upon scale, but for the most part it is agreed they are able to possess a human and control them. If anything, demons are often strategic and tactful in what they do, but are always up to no good.