When you work out, you are exercising specific areas of your body, and certain moves are designed to target different areas.  Working out your shoulders is done best by exercises like overhead presses and arm circles, and working out your hamstrings are done best by deadlifts and flutter kicks.  Your biceps are best targeted by barbell curls, and sit-ups are designed to work your ab muscles.  For the best results, be sure to vary your workouts so you engage all areas of your body and make sure you are performing the moves correctly.

Anatomy of Fitness - The Appropriate Workouts for Different Parts of the Body