Satan, once known as Lucifer, was a guardian cherub of God and is now a fallen Angel. He had became entranced by his own beauty, power, and wisdom, and had convinced himself that he was greater than God. Satan tried to overthrow God by amassing an army of angels against their Creator, but God knew his evil plans from the very beginning and cast him out of his holy mountain.
Jesus Christ referred to Satan as the prince of this world, and Paul said he is the prince of the power of the air as well as the "god" of this world. The best description of Satan might be in the names attributed to him in the Book of Revelation; which calls him the dragon, and that old serpent from the Garden of Eden, also called the Devil, even Satan.