Among strength exercises, the pushup should definitely get a gold star, since it's easy to do and works a number of different muscles of the upper body. It is possible to get "buff" doing just this one exercise, but you'll have a much better chance if you incorporate other types of exercise as well.
What You'll Target
You might feel those pushups primarily in your chest, but the exercise targets more muscles than that. Pushups target the sternal portion of the pectoralis major -- the bulky portion of the pecs that inserts onto the sternum. The exercise also recruits the deltoids of the shoulders, the triceps and biceps of the arms, the clavicular portion of the pectoralis major and the abdominal muscles, including the obliques and rectus abdominis, or "six-pack" muscles. Another thing to keep in mind -- while pushups are going to help strengthen your muscles, if those muscles are hidden by a layer of fat, pushups alone are not going to make them magically appear. To lose fat, you have to do calorie-burning exercises, such as running, walking or any other activity that gets your heart pumping.
Test Your Mettle
One of the important tenets of muscle training is the need to work your muscles to fatigue. When you work your muscles until they're exhausted, you'll have a better chance at gaining muscle and building strength. For weight training, you do this by making sure you're lifting the proper amount of weight. For pushups, however, you'll have to work to fatigue by doing the right number of repetitions. When you start out with your training routine, test yourself by getting into the proper pushup position -- either in a plank position for men, or with the knees resting on the floor for women. Place a rolled-up towel under your chest area, and then do as many pushups as you can, meeting your chest to the towel each time. Record that number and then use it as a baseline for your future workouts.
Design a Workout
If you're hoping to gain muscle and get buff by doing pushups, set aside time in your schedule about three to four days a week for training. Like other muscles, the upper-body muscles need time to rest and generate new muscle tissue. Thus, you should give yourself 24 to 48 hours between pushup sessions. Do a short warm-up of walking, jogging or cycling for about five minutes before your pushups. Then complete a set of pushups, aiming to do one more repetition than you did during your test. Take a short break and then do a second set until your muscles become fatigued -- you may or may not be able to complete a full second set. To further help you in your quest, make sure you're getting adequate protein and proper nutrition. Adults need a minimum of 0.4 to 0.5 grams per pound of body weight per day.
Keep Your Muscles Guessing
Over time, your muscles are going to adapt to the work you're doing, and you'll need to add more repetitions to continue working your muscles to fatigue. Try adding more repetitions every one to two weeks. Variation in the types of exercises you're doing will also help in your effort to get a more toned upper body. Try doing pushups with your hands resting on a elevated platform, doing one-handed pushups or pushups with a clap in between to make the exercises more challenging. You might also consider a workout routine that involves a few more upper-body exercises, which you can cycle through, such as pullups, bench presses or other weight-lifting exercises using a set of dumbbells.