This is a decent tutorial if you can handle the horrible music that is played rather then narration. Displacement as many of you probably already know can be a really powerful tool for making complex geometry using only maps. Like the stone in this video, you can get the effect of modelled stone with just a map applied to a plane, like a bump map, except displacement actually creates the geometry of the stone based on your map...so it looks more convincing.

Anyway...it can be kind of tricky so check out the basics of how to do it here in this tutorial.