Yes, the devil certainly does exist; he isn't just an idea people dreamed up to explain the existence of evil. Someone has said that one of Satan's tricks is to convince us he doesn't exist -- but he does.
Repeatedly, Jesus referred to the devil as a real being. At the beginning of Jesus' ministry, the devil tried to divert Him from God's path; at the end of His ministry, the devil prompted Judas to betray Jesus. Satan is a powerful spiritual being whose primary goal is to turn us away from God. No wonder the Bible warns, "Do not give the devil a foothold" (Ephesians 4:27).
Although he is a spiritual being, Satan apparently can assume human form on occasion. However, the Bible doesn't tell us what he might look like; pictures of the devil with a red suit and pitchfork are simply products of someone's imagination. At times, the Bible says, he may appear as a very attractive and winsome personality; the Bible says he even "masquerades as an angel of light" (2 Corinthians 11:14). At other times, however, his evil, vicious nature is obvious.
Never forget two facts about the devil. First, he is your enemy. He'll do everything he can to deceive you into thinking his way is best -- but it isn't, for it leads to hell. The Bible says, "Your enemy the devil prowls around like a roaring lion looking for someone to devour" (1 Peter 5:8).
Second, never forget that Satan is a defeated foe! By His death and resurrection Jesus Christ overcame Satan's power, and some day his defeat will be final. Are you on the winning side? You can be, by giving your life to Christ.