The word devil is rooted in the ancient Greek term diavolos, which translated into English means accuser or slanderer. This figure is represented not just in the Abrahamic religions of Judaism, Christianity, and Islam, but also in various other religions around the world. In most religions the devil is an ultra powerful supernatural being, the whole personification of all evil, and the enemy of both mankind and God. The devil is regularly connected by his nature to infidels, unbelievers, criminals, heretics, and pagans. Of course within Christianity and Judaism, the devil is simply another name given to Lucifer or Satan, the notorious fallen angel who rebelled against God, and was ultimately cast out of heaven by the mighty archangel Michael. Some regard the devil as a literal spiritual entity that actually exists, while other see him as an allegory to a crisis of faith, free will, individualism, wisdom, hope, love, connectedness, or spiritual enlightenment.
With regard to standard Christian dogma, it is widely believed that the Devil and God are perpetually fighting for the souls of mankind, with the respective destinations of each force being either heaven or hell. It is also thought that the Devil commands legions of demon or evil spirits, that not only wreak havoc amongst the living, but regularly lure humans into committing sins against God. The Christian belief in the Devil originates from the Old Testament scripture that describes the serpent that tricked Eve into sinning, thus setting into motion not only sin itself, but also it's direct fruit, which is death. The Devil or Satan, develops as the Old and New Testament of the Bible unfolds, with such epic endeavors as his task of trying to tempt Jesus Christ into sinning. From start to finish the Devil shows up in various portions of Biblical scripture, with the Book of Revelation finally laying out a prophecy regarding the Devil's final end and demise.
The concept of the devil is quite common even outside of religious beliefs, where even some atheists tend to label anything bad as a work of the devil. Whether it be a figure of speech, or the personification of all things wrong with the world at large, the concept or idea of the devil is very much alive and well amongst most people and societies, in one form or another. Unfortunately the devil concept is also used to personify not only the evils of mankind, but also it's differences. For example it is still quite common even in the contemporary world for races, cultures, social groups, or economic classes to label each other as the devil, or spawns of the devil, and sadly it is often meant in the literal sense. Outside of any organized religion, it could probably be concluded that the concept of the devil was one of the very first concepts to be defined by mankind, and it undoubtedly dates back to well beyond even basic human civilizations.
A concept that is somewhat unorthodox when compared to more mainstream beliefs, is that the devil and God are one in the same, or two faces of the same deity. People that subscribe to this notion often reject the age old notion of duality, such as good verses evil, or good deeds verses sin. Other unorthodox belief systems include devil worship, or Satanism, which basically entails worshipping the devil in place of God, for various purposes and reasoning. It is often thought that worshipping the devil comes with certain extra human powers, which are accepted in trade for selling one's soul to the devil. There are blood rituals and a whole host of bizarre symbolism attached to devil worship, some of which play into or focus upon anything that could be considered as being unacceptable by any particular mainstream social or religious system, a sort of rebellion against all social norms. Lucifer the rebel without a cause, the James Dean of Hades.