A demon is an evil spirit. Demons are mentioned several times in the Gospels, usually in the context of having possessed people and being driven out of them by Jesus; in most cases, the victims of possession have symptoms that would be given other explanations today such as mental illness or epilepsy, although this is not to say that those specific cases were not caused by demons. False gods mentioned in the Old Testament, such as Baal, are considered by some Christians to have been demons, as are the pagan gods of Greek, Egyptian and other mythology.
Like angels, demons are seldom physically described in the Bible. Consequently, artists have a lot of licence to use their imagination when representing them, typically giving them grotesque and ugly features such as leathery wings, pointy teeth, staring gimlet eyes, claws, forked tongues and tails, hairy feet and oversized ears. Demons may also be strongly associated with physical phenomena such as fire, or human emotions such as anger or bitterness. In fact, anything that is perceived to be negative or destructive may be explained by or associated with demons, although whether these demons are metaphorical or real, and in what sense, is intrinsically difficult to establish.
The term demon originally derives from anicent Greek daimon. In the ancient Greek religion, daimons were spirits which existed between the level of humans and the gods, and mediated between the two - carrying the prayers and sacrifices of humans to the gods, and carrying messages from the gods back down to humans. The Greeks believed there were both good daimons - calodaimons - and others were evil - cacodaimons. For example, in Plato's dialogue The Symposium, Socrates calls Love a daimon, by which he meant it was less than a god but greater than a human being. With the coming of Christianity, however, usage changed - the term daimon took on a decidely negative tone, being used to refer to evil spirits only. Spirits which were perceived as good were known instead by the term angels. However, a few Christian authors continued the original Greek meaning daimon, such as Pseudo-Dionysius the Areopagite, who used the term daimon to refer to the angels of God.