Push-ups strengthen your pectoral muscles.
Novice bodybuilders often hone in on the pectoral muscles - that is, the chest muscles - as one of the most obvious, and impressive, targets. But your chest muscles are there for much more than show, and they assist with a surprising variety of movements that you execute in sports, the gym, and everyday life.
Know Your Anatomy
There are actually two notable muscles in your chest: The pectoralis major is by far the largest and the easiest to see, and it's the muscle most people think of when it comes to the chest. But you also have the pectoralis minor, a smaller muscle that helps connect your ribs to your scapulae, or shoulder blades.
Understand Their Function
When taken as a whole, your chest muscles perform a surprisingly complex set of functions, acting on both the shoulder blades and your shoulder joint, which is itself very versatile since it can move in almost any direction. Your muscles rarely act in total isolation from each other, but that's especially true of the shoulder joint and shoulder blades, where multiple muscles are activated to make any given movement happen.
With all that said, the easiest way to make sense of your chest muscles' function in the body is to focus on the movements where they are the primary movers, or the muscles that provide most of the "oomph" to make the motion happen. Those are mostly pushing movements, whether you're pushing anything out in front of you, down below you, or - to a somewhat lesser degree - up above you.
See the Pecs in Action
In everyday terms, the pectoral muscles help with motions like pushing a bag of groceries across the counter, holding a baby up overhead or pressing yourself up from the ground or the bed. Even though they're not exerting a lot of force, your pectoral muscles are also active - along with other arm and shoulder muscles - when you push the hand brake in your car down, throw a ball (a somewhat less obvious interpretation of the "pushing" motion), move a stack of papers across your desk and push a grocery cart or stroller.
Push Up and Press
The general rule for exercises that strengthen any muscle is that they should mimic the motions where that muscle is used, with the addition of extra resistance to challenge your muscles so they become stronger or build better endurance.
Push-ups are one of the best chest exercises, because you don't need any special equipment. Dumbbell and barbell chest presses mimic the same movement, using free weights instead of gravity for resistance. You can do push-ups and chest presses at an incline or decline to fine-tune which part of the muscle you target.
Try the Fly
Of all the free weight exercises you can do, the chest fly or pec fly is the best at isolating your pectoral muscles. At its most basic, this exercise involves lying flat on your back; extending your arms straight over your chest, palms facing in and barely bent elbows pointing out; then spreading your arms apart as if they were the covers of a book. Once your elbows reach shoulder level, reverse the motion to complete the repetition. Some gym weight machines - variously called the butterfly machine or the pec deck - allow you to do this motion while sitting up.