The origins of the belief of demons among the ancient societies of the world have been shrouded in mystery and it is uncertain just exactly where the concept started, though a good start would be with the Persian and Assyrian empires... some of the most ancient civilizations that we are aware of today.
Nearly every culture that has been recorded in the history of mankind has had some form or variation of demonic belief. Today the most widely recognized aspects of demonology are derived from Judaic, Christian and Catholic sources. Many archaic stereotypes presented within these religions, as well as the prolific historical inaccuracies long associated with fictional print and media, persist in spite of evidence to the contrary.
Most people today imagine short horns, leathery bat-like wings, long pointy tails and menacing pitchforks to be the common accepted image of demons in general, and Satan in specific. However, this is a far cry from how demons were presented in the past.
Initially, in many of the most ancient cultures of the past, demons were seen as having the capacity for both good and evil. The origin of the word demon, in fact, may have come from Indo-European sources and simply meant 'Celestial Body'.
There has often been confusion when regarding Devils and Demons in many Orthodox traditions. A Devil is an entity who usually fights a goodly and divine authority for control of existence. Many such battles will be settled in an 'End of Time' scenario where the forces of good vanquish the forces of evil.
Demons are much like Angels in that they are lesser beings who are usually under the authority of the prevailing Devil. They are always malevolent in nature, and seek to torment humans with their dark and wicked ways. The means by which they wreak havoc are threefold:
#1: In spiritual form they whisper evil thoughts into a person's mind, trying to corrupt them to do wrong.
#2: In spiritual form they restlessly wander the earth and torment others with their mere appearance.
#3: They actually possess someone and control their actions, usually causing them to harm others, or at the very least, themselves. This is called 'Possession', and even today it wreaks fear in many just by even mentioning it.
Another thing that is usually representative of demons in nearly every culture; they are usually grotesque in appearance, and more than likely half animal at least.
Perhaps one of the greater influences in restoring interest in demons and their study in the mid-20th century was Anton LaVey, the author of the Satanic Bible and the founder of the Church of Satan. LaVey believed that Christianity was a repressed religion, and the true power of Satanic belief was the lack of limits imposed on the believer. LaVey believed that man ought to have the supreme choice over his life, not a God, or any god.
Damned by the gods; feared by man; demons have made their home on earth, and they're here to stay.