Apocalyptic writings historically appeared between the years 200 BC and AD 100. They were said to be inspired by an angelic mediary, who showed the writer various visions that revealed God's purpose in history. Mostly, apocalyptic writing focuses on a dualistic power situation with good vs evil, and also sought to explain why God's faithful were experiencing suffering whilst offering them future hope of salvation and peace. The major purpose of such writing was to reassure the readers that everything was under control and in God's hands, even though it appeared that the church was suffering a great deal at the time. There was always the promise of God intervening in history, acting to save his people and a happy ending. Note how the apocalyptic nature of the Book of Revelation is indicated in the opening two verses:
The revelation from Jesus Christ, which God gave him to show his servants what must soon take place. He made it known by sending his angel to his servant John, who testifies to everything he saw—that is, the word of God and the testimony of Jesus Christ.
John here openly acknowledges that the "revelation" he shares and the visions he had were given to him by God through an angelic emissary, and that the purpose of this angelic visitation was to "show his servants what must soon take place." This book is therefore apocalyptic through the way in which it seeks to show how the suffering of the present maps into the overarching plan of God and how it aims to give comfort to God's faithful through a revelation of what will happen in the future and how God will act to save his people.