The author of the book of Revelation is a man named John, who was a Christian exiled to Patmos because of his faith. The author does not claim to be John the Apostle who wrote one of the Bible's four gospels.
Some of the founders of the Christian church believed the author was John the Apostle, but it is unclear whether the author actually was the apostle or one of his followers. Scholars who doubt that John the Apostle authored Revelation point out that the writing style and vocabulary of that book differ from what is used in the gospel of John. The fact that the book of Revelation is written as a letter to seven early church communities indicates that the author was seen as a leader by those churches.