Jonathan Williamson creates great videotutorials - they're always easy to follow and listen to (I always find a good voice and good pace are always paramount for a good videotutorial). Today, he explains the retopology tool in detail.
The retopology tools in Blender allow you to take a high-resolution model and very quickly create a low-poly version of that model. You can then use this new low-poly mesh to easily generate a normal map, displacement map, or anything else you may need from the high-poly version.
This tutorial introduces you to the basics of retopo in Blender, including how to use the extrude feature, the paint tool, and some of the quirks of retopo in general. Retopologizing a model.