A demon is a generally evil creature often personifying human torments. The term demon is often used synonymously with said torments. (TXF: "Fire") The study of demons is referred to as Demonology. (TXF: "Sanguinarium")
According to the Bible, demons are Satan's angels. They were originally God's angels, but have joined Satan (formerly Lucifer) in his rebellion against God and have been cast out of Heaven. They are actively working against God, his angels.
Demons could be responsible for a number of paranormal phenomena such as unexplained noises, apparitions and objects moving on their own. Demons could remain in places where Witchcraft, transcendental meditation, séances and other occult activities have been practiced and trouble the inhabitants (see “Blumhardt’s Battle: A Conflict with Satan” by Johann Christoph Blumhardt). They could also be responsible for manifestations attributed to ghosts. Possession is essentially linked to demonic activity.
Many demons take on the form of human beings in their pursuit of tempting people. In the Middle Ages, demons often would appear in demonic form prior to tempting souls. However, by the 20th century and the advent of psychology, their demon forms did not have the desired effect, and appearing as demons would often create a condition where the person being tempted believed themself to be insane. (MM: "Somehow, Satan Got Behind Me")