it works out ur whole leg
squats work out ur entire leg. Mostly ur quad, but ur hammy, buttocks and calf as well.
if you're an athlete theyre great because so much power comes from you legs. they also help with speed and vertical jump
if you're a body builder then...well...you'll have nice legs.
forgot to mention...they help with balance as well, which is one of the most important things...ever...