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.
It should be no surprise that Revelation is a book of prophecy. If you have heard about the book of Revelation at all, you have probably heard that it predicts the future. It stands at the heart of teaching about the "end times" such as we find novelized in Tim LaHaye and Jerry B. Jenkins' series, Left Behind. 
And it calls itself a "scroll of prophecy" (e.g., 22:19). Some versions read "book of prophecy" here but the NIV translators wanted to make clear that, when Revelation was first written, it was not in a book like we buy books at a store. It was on a scroll.
It is no surprise that when we read the Bible today, we define the words the way we use words today. So when we see at the end of Revelation that those who add or take away from "the prophecy of this book" (22:18, ESV), we will automatically assume these words were about the whole Bible. After all, these words come at the end of the Bible as it comes packaged at my store. But these words originally referred only to the scroll of Revelation, as the new NIV puts it.
Here is a big warning. Individual Christians are often highly convinced that they know what God thinks about just about everything because they own a Bible. But we are so used to just reading the words the way they strike us in English that we don't even realize how little we may actually know about what the Bible really meant. Knowing the Bible is not just a matter of being able to quote its verses. There are plenty of pastors who can quote countless chapters and verses but who have no real clue what those words actually meant when they were written two and three thousand years ago.
So Revelation 22:18-19 was originally a warning to those who might add or take away from the prophecy of the book of Revelation itself. John had no idea that his revelation would end up at the end of the whole Bible.  The process of the books of the New Testament being collected into the book form that we now have took several hundred years, and Revelation was one of the books that faced more debate than others over its inclusion.
We have talked a little about the genre of an apocalypse. What was the genre of prophecy like? Many people are prone to see prophecy in the Bible as a matter of what is going to happen today. For example, when I was a boy, the big thing in prophecy was a book by Hal Lindsey called, The Late Great Planet Earth.  Written during America's Cold War with the Soviet Union, prophecies about places like Gog and Magog suddenly came to be about a coming conflict between the United States and the Soviet Union.
It is only because we have a short attention span that we do not quickly realize how pointless it is to try to match biblical prophecy with current events. A certain segment of Christianity has been doing this sort of thing for almost two hundred years now, and they have obviously been wrong on every prediction so far. Whole churches have been formed around some individual's predictions about how the end times were about to play out.
On the one hand, it would be wrong to say that prophecy did not often involve an element of prediction. After all, the prophet Agabus in Acts 11:28 predicts there is going to be a famine and, sure enough, there is a famine. But even more than fore-telling, prophecy was usually much more about forth-telling, giving a message from God to the people right in front of the prophet. Prophets brought "a word from the Lord" to the people in front of them, a word the people needed to hear, just like preaching today. It was often a word to get them back on track, a corrective word.
Even when prophecy was about the future, it was primarily about the immediate future, rather than the far off distant future. True, the New Testament authors often heard predictions of their own day in the words of the Old Testament prophets. However, a close examination of these fulfillments often reveals that the New Testament authors were hearing newly inspired meanings to the words rather than the words as the prophets first understood them themselves. 
Just to give an easy example, when God first gave the words of Hosea 11:1-2 to Israel, he was condemning Israel for serving other gods. "When Israel was a child," God says, "I loved him." Indeed, "out of Egypt I called my son." God here is clearly reminding Israel of the exodus. But rather than serve him, "the more they were called, the more they went away from me. They sacrificed to the Baals and they burned incense to images" (11:2).
The first meaning of these words was obviously part of a prophetic word to the Israel right in front of Hosea. It was condemning them for serving other gods even after God had done so much for them.
Imagine our surprise, then, to find that Matthew heard in these words a meaning about the Messiah leaving Egypt as a child (Matt. 2:13-15)! This is a meaning we can doubt a single individual heard in these words before the time of Jesus. No doubt everyone thought the meaning of these words was completely understood in reference to the exodus of Israel and its subsequent disobedience.
Hosea 11:1-2 is not in the form of a prediction; it is already in its day about the distant past of Israel. It says nothing about a Messiah, a concept that did not yet exist at the time because not only Israel but Judah still had a king and weren't longing for a promised king to restore the kingdom. Still more, Hosea is not about anyone's geographical travels from point A to point B.
But this is not a problem. It may initially seem a problem because of incorrect expectations we might have. But it is not a problem for Matthew, who is interpreting Old Testament texts the way that the Jews of his day interpreted biblical texts.  Matthew is hearing an inspired meaning in the words of Hosea, even if it was not a meaning that Hosea or anyone else in Israel had heard in those words before Jesus.
The point is that many of the prophetic understandings of the New Testament authors were spiritual readings of the Old Testament, understanding the words in a "fuller sense."  The original prophecies of the Old Testament were much more about giving a word to the people right in front of them rather than a word to those who would live even a few hundred years later, let alone to us two thousand years later. So we should, in the first place, expect the words of Revelation to have made sense to the people to whom it was first written. We can speculate about whether the Spirit might want to relate these words to current events, but we should be very cautious and remember that all such speculation thus far seems to have been wrong.
[insert textbox on four approaches to the prophecy of Revelation: futurist, historicist, preterist, and idealist]
So Revelation begins by telling us that it is about "what must soon take place" (1:1), confirming that its first audience were people alive in John's day. Indeed, the third genre of which Revelation partakes is that of a letter. It was a letter to seven actual churches in "Asia," which at that time referred to the western part of Turkey. We now turn to the concrete messages that Jesus spoke to these churches through the mouth of John while he was exiled on the island of Patmos...
 The first volume of the series was Left Behind: A Novel of the Earth's Last Days (Carol Stream, IL: Tyndale, 1995).
 Of course God knew, and you could argue that God intended these words to end up at the end of the Bible. It is a fair proposition. However, when we argue things like this, we should keep in mind that we are going beyond the Bible to say so, since the Bible did not mean this, and most of the books of the Bible are not in the genre of prophecy.
 Hal Lindsey, The Late Great Planet Earth (Grand Rapids: Zondervan, 1970).
 Again, God of course knew that the New Testament authors were going to hear those meanings in the words, so we can suggest that God implanted some words in the Old Testament with a potential double meaning that he knew he was later going to inspire the New Testament authors to hear.
 For a thorough study, see Richard Longenecker's now classic, Biblical Exegesis in the Apostolic Period 2nd ed. (Grand Rapids: Eerdmans, 1999).
 Or a "sensus plenior," as it has been called. Bible school teachers in my own revivalist tradition, like my own grandfather, used to make a distinction between the "near" and the "far" of prophecy.