The push-up is probably the most recognizable upper body exercises. While the chest is the main target in the standard push-up, it actually works just about all upper body muscles to some degree. Here's how it's done:

First, kneel down on the ground and place your hands shoulder width apart. The body should be suspended on four contact points: the toes of both feet and each arm. Next, ensure your back is perfectly straight, then slowly lower your upper body until the chest is a few inches from the ground (or until the elbows are 90 degrees). Slowly push up to the starting position and exhale as you push. Repeat the exercise.