http://www.exrx.net/WeightExercises/OlympicLifts/Clean.html
It is really a full body exercise so to speak. However, I feel the primary benefit is the development of explosive power, particularly with the hips.
A triple extension is involved: hips, knee, and ankles. This is where the power of the pull comes from, not the arms. The arms do nothing but guide the bar and allow your to "wrap" under it. You do top off the triple extension with an explosive shrug, so this exercise also does wonders for the upper back. Of course, you finish it with a front squat, so your core and quads get butt-raped too.