In Christianity, the Devil is named Satan, sometimes Lucifer. He is a fallen angel who rebelled against God. According to Islam he is a genie, since angels had no free will and could not fall. He is often identified as the serpent in the Garden of Eden, whose persuasions led to original sin and the need for JesusChrist's redemption. He is also identified as the Accuser of Job, the tempter of the Gospels, and the dragon in the Book of Revelation. The Devil is the author of lies and promoter of evil, however he can go no further than the word of God allows, and temptation is likely to come from original sin rather than Satan tempting people.
Some Christians believe that Satan isn't an entity, but a personification of Sin, since he wasn't written about until the New Testament.