who are the fallen angels? Why they have turned from angels to fallen angels?

Called Lucifer in heaven before the his disobediance, Satan is one of God's favorite angels until his pride gets in the way and he turns away from God. Satan brings many of heaven's angels with him, however, and reigns as king in hell. He continues an eternal battle with God and goodness for the souls of human beings. Satan, at first, is an angel with a single fault, pride, but throughout the story he becomes physically and morally more and more corrupt.