In the creator's words: "Probably the easiest way to describe Spelunky is that it's (kind of) like La Mulana meets Nethack - every time you play the levels, items, monsters, and so forth, are all procedurally-generated. And the terrain is destructible and there are quite a few ways in which the various game elements can interact with one another.
My goal was to create a fast-paced platform game that had the kind of tension, re-playability, and variety of a roguelike. In roguelikes, the gameplay tells the story, and I wanted to give Spelunky that type of a feeling... but make the player rely on their reflexes rather than their brain (or knowledge of what 50 billion command keys do!). If there's a best of both worlds, that's what I was trying to go for."
Basically, it's super awesome, and you should play it now.