If you saw one of the latest articles on Blendernation, The Retopology Tutorial, or if your working on a game with with the new GLSL settings then you'll probubly find this very useful.
Once you've created a low poly and high poly version of a mesh object you have the ability to make normal maps from the high poly (possibly sculpted) object and then 'bake' it onto the low poly model. This 'normal mapping' technique is very effectively used for games to get extra detailed looking objects and characters.
The tutorial is nice and simple, with easy to follow steps. Its also good to have something like this around to refer back to, in the case a step or two have been forgotten.