Muscles developed: Triceps, Deltoids, Pecs and Lats
Everyone needs to be doing pushups whether they are new to strength training or an avid lifter. I believe push ups and pull ups are two of the best exercises you can do.
They can be done anytime, anywhere. In my high school strength program we don't even allow an athlete to start using the bench press until they can complete 30 push ups in one minute.
Proper Form
Lie flat on the ground facing down.
Place your hands on the ground slightly wider than shoulder width.
Put the weight of your lower body on your toes.
Keep your body perfectly straight and stiff.
Raise your body by straightening your arms.
Hold at top for one second.
Slowly bend your arms and lower your body.
Keep your head facing the floor.
Stop lowering your body when your chest is 2-3 inches from the floor. (Some people place a dictionary under their chest so they know that they have lowered themself far enough when their chest touches it.)
To lessen the resistance you can bend your knees and allow them to support the weight of your lower body instead of your toes.
To increase resistance you can elevate your feet on a bench or chair.
Changing hand positions will effect different muscles in the area and is encouraged. I have my athletes hold the handles of dumbbells for variety.
Push ups can be done everyday. Do many as you can. Work to increase your repetitions on the next workout.
This is a really good video showing how to change the push up to suite your needs.