The book of Revelation begins by calling itself, "the revelation from Jesus Christ" (1:1).  The Greek word for revelation is apocalypsis, which is why the book is sometimes just called, "The Apocalypse."
In our chapter on Jude, we saw that "apocalyptic literature" was a body of Jewish literature from before and after the time of Jesus that had to do with revelations about the heavens and what was going on there.  Angels usually featured heavily in this literature, usually bringing these revelations down to the earth to people like Adam, Enoch, Abraham, Moses, and so forth. The book of 1 Enoch, which we discussed in the chapter on Jude, is about revelations that various angels brought to Enoch not only about the future but also about the spiritual geography of the world.
The kind of apocalypse we are especially interested in, then, was a type of literature where a heavenly being, usually an angel, brings a revelation about events that are going to take place in the future.  Most Jewish apocalypses used a clever literary device to write about events going on in their own time. The anonymous author would tell about an angel appearing to some authority figure in the past and have them predict the future up until the time when the book was actually being written.
For this reason, it is usually fairly easy to date the writing of a Jewish apocalypse. The history will be very exact until it gets to the actual time of writing. At that point, the prediction usually jumps to the end of history or begins to go off. You can see why these apocalypses tended to be written during a time of significant turmoil or distress, because an apocalypse was a way of giving hope when the visible world around the Jews did not look very promising.
An apocalypse basically said to its audience that while things might not look great on the earth, God was still in control in the heavens. Parallel to the battles being fought on earth were battles being fought in the heavens as well. The heavenly was about to erupt into the earthly and without any question, God would win.
While the book of Revelation shares some of these features, it is also unusual for an apocalypse. For example, most would accept that John actually was its author (1:4). Tradition holds that this is John the son of Zebedee, one of Jesus' original disciples. I personally see no reason to question his authorship.  It is of course possible that the literary genre of apocalyptic literature has influenced the way John's visions have been presented. More on this later.
So the book of Revelation is an "apocalypse." It also partakes of two other genres of literature. It is also a letter and a prophecy...
 Many people mistakenly call "Revelation" (singular) the book of "revelations" (plural), but while it is true that there are multiple revelations in the book, it is more properly just called "Revelation."
 John Collins defines apocalyptic literature in this way: it is "a genre of revelatory literature with a narrative framework, in which a revelation is mediated by an otherworldly being to a human recipient, disclosing a transcendent reality which is both temporal, insofar as it envisages eschatological salvation, and spatial, insofar as it involves another, supernatural world." See The Apocalyptic Imagination: An Introduction to Jewish Apocalyptic Literature, 2nd ed. (Grand Rapids: Eerdmans, 1998), 5.
 Sometimes called a "historical" apocalypse, in contrast to the more "travel" type apocalypse that reveals the spiritual geography of the world.
 Remembering that, in the previous chapter, I followed those who think the author of the Gospel of John was likely a different person than John the son of Zebedee.