Satan
Satan (from the Hebrew "ha-satan", "accuser") was a member of the divine council of God referenced in the Old Testament in Job. He is referenced 11 times in Job and 4 other times in the Old Testament. The New Testament mentions satan 34 times in all starting with the first book Matthew and ending with the last Revelation. In the Christian tradition he rebelled against God and is now the enemy of God and man alike. He is the father of lies and no good is found in him. He leads a host of fallen angels (or demons) who know their days are numbered before they will be cast in the lake of fire and they seek to take as many humans as possible with them. Those who worship Satan are called Satanists.
History and Goals
In Christianity Satan's goal is to lead people away from the love of God, by tempting or tricking them. The only sources of supernatural power in the world are from either God (good) or Satan (evil). In the book of Genesis, Satan takes the form of a snake and tempts Eve with fruit from a tree. This causes sin to come into the world. Satan, in the Book of Job, listens to God speak highly of Job and accuses Job to God before the host of angels that Job praises God only because he is blessed and would curse God if he was forced to suffer. God allows satan to do what he will except to spare his life, and Satan causes Job to suffer immensely. (Job remained true to God through his hardships; Satan accused him further and punished him further, but could not get Job to break).
Jesus makes references to demonic possession on multiple occasions that He then casts out. Some believe that Satan is able to possess and control living humans on Earth even today. The Catholic church believes that priests (and sometimes only bishops) are able to exorcise this possession through Jesus.
In the New Testament, Satan tries to tempt Jesus in the desert, and fails. Jesus warns extensively to beware of Satan as does Paul.
Satan, is also called by several different names (the Devil, deceiver, Bealzebub, Beelzebul, Lucifer, Asmodeus, Abaddon, "Old Scratch," the Crooked One, etc).
The notion of Satan, or an evil force, is noted in many if not all of the major world religions.
Jewish Views
In the Jewish tradition, Satan is an angel who faithfully serves God as a prosecuting attorney - one who accuses men of wickedness and impiety. At the direction of God Satan may be permitted to test these accusations, such as in Job. In this view, Satan's goal is not to lead men away from their faithfulness to God, but merely to reveal the true depths of their devotion, although in I Chronicles 21:1 Satan provoked David to take a census of the people of Israel against the will of God and 70,000 men are slain because of it.