A Demon is an unholy or unnatural being. Demons are usually, but not always, malevolently evil.
Christian Demons are considered to be either Fallen Angels or the souls of the human dead. They are God-haters and can appear in a variety of different forms including as humans, cats, and goats. Christian Demons have the ability to posess people. This posession can result in the host foaming at the mouth, screaming obscenities against God, vomiting strange objects, and posessing super-human strength. Demons must be driven out of their host through exorcism.
The Egyptians, Chinese, Koreans, and many other ancient cultures also had their own forms of demons.
Traditional ways of keeping demons away from you include: hanging green branches over the doorways leading into your home, scattering bean flowers through the house, wearing a necklace made of peach pits, and holding a tamarisk branch and scattering the leaves.