Apocalypse and Epistle
Revelation actually falls into two genres—it's both an apocalypse and an epistle. Talk about doing double duty.
An apocalypse is basically just a revelation (Revelation!)—i.e., revealing something unknown. When we hear the word today, we think of it as a story about the end of the world. No big deal, right? It typically involves symbolic visions of future events or even magic carpet rides up to Heaven (magic carpet not required).
The whole purpose of the story is to tell about a world that is so gripped by evil that God needs to come in an save it. God's solution for defeating evil often includes unleashing angels of death and destruction, so they're typically not warm and fuzzy little tales. But God needs to get this info out to people before it's too late. Once the rivers of blood start flowing, there won't be as many chances to repent (source, p. 1287).
An epistle, on the other hand, is just a fancy name for a letter. John has actually written a circulating letter; that means the letter is passed around to all different communities of Christians. It's sort of like what happens in the book The Sisterhood of the Traveling Pants except Christians are sending around stories of plague and destruction instead of a pair of friendship-enriching jeans.