In Christian mythology, a demon is an angel who has fallen from grace along with Satan. Demons are inherently evil and frequently plague humanity under the command of the Devil.
Demons are supposedly able to enter and take control of a human in a process called possession. The Catholic Church uses the ritual of exorcism to expel demons from the victims they possess.
Demon possession used to be a common explanation for those who suffer from neurological disorders such as epilepsy or schizophrenia.