The word Demon derives from the Greek term Daimon and means simply a spirit of some kind - not necessarily a bad one. However, the word has acquired a distinctly negative connotation since then. Nowadays, most people use the word Demon to refer to evil, hellish spirits or beings. Some religions describe Demons as "Fallen Angels", i.e. Angels who rebelled themselves against God.
They have been most often portrayed as horned humanoids, usually small ones - but can also assume several forms depending on the work they appear in. They are also typically capable of possessing Humans, as well as other species, objects and even locations.
Many species in science fiction are referred to as demons, resemble the mythological interpretation of a demon, or are in fact some form of demon or demonic entity: