In light of a new Bible study our church is doing on the book of Revelation, I have been thinking through the difference between Prophesy and Apocalyptic as genres of literature. It seems to me that these two very different but often related genres have two different points. The point, as I see it, of Prophesy is a call to repentance; it often maps out what's going to happen unless there is repentance. However, the point of Apocalyptic is a call to endurance; "this is what will happen, but even so, persevere and endure in faith."
A professor once summarized it like this: "prophesy says 'the world is coming to an end unless...' and apocalyptic says, 'the world is coming to an end...'" This is basically true depending, of course, on what one means by "end." In Revelation, for example, "end" does not mean ceasing, as in destruction, rather, the "end" of the world in Revelation is the world's renewal and its being placed back on the right track as it has longed for since Genesis 3--"I am making all things new" (Revelation 21:5).
Apocalypse often contains a kind of prophesy, namely what we'd call "ex eventu prophesy", a post-event prophesy which describes an event which has already happened as though it is being foretold. Ex eventu was often used as a literary devise to give credibility to the foretelling of future events (as in the Animal Apocalypse of 1 Enoch 85-90. See Michael G. Reddish, Apocalyptic Literature: a Reader, page 43).
the greatest difference between Prophesy and Apocalypse lies in their purposes. As I said before, the purpose of prophesy is to get people to change their future. But the purpose of Apocalyptic literature is to provide hope to the oppressed or those in a crisis by offering an alternative picture of what's going on and to provide a lens through which they might interpret their experience that will give them hope because they have seen where this all is headed. It gives hope, not fear, in order for oppressed or suffering people to be able to carry on. This is a purpose which is largely ignored by dispensationalism.