I think I heard somewhere that Satan was originally an angel. But if that's true, is he still an angel, or what happened to him? Or is Satan just an idea that people dreamed up to explain the existence of evil?
No, Satan is not imaginary; he isn’t something people dreamed up just to try to explain the existence of evil. Satan is real, and the Bible makes it clear that he is a powerful (and evil) spiritual being who is absolutely opposed to God and His goodness.
The Bible doesn’t give us a detailed account of Satan’s origin but it does indicate that Satan originally was one of God’s angels, created by Him to carry out His will. But apparently Satan became filled with jealousy and pride, and he decided that he would lead a rebellion against God so he could take God’s place as the ruler of all creation. The Bible says, “You said in your heart… ‘I will make myself like the Most High'” (Isaiah 14:13-14).
But Satan’s rebellion failed, and he was cast out of heaven. Now Satan and those who followed him are fallen angels—no longer part of God’s heavenly army, but separated from heaven’s glory forever. He still is powerful, however, and he still does everything he can to block God’s plan for our lives. Don’t be deceived; Satan is your enemy, and he will do everything he can to keep you from God.
The greatest truth about Satan, however, is this: he is a defeated foe! By His death and resurrection Jesus Christ conquered the forces of sin and death and hell and Satan. Are you on Christ’s side? Make sure, by turning to Christ in repentance and faith, and committing your life to Him for your salvation.