Pushups are a compound exercise meaning they target multiple muscles at the same time.  Pushups elevate heart rate so you're getting cardio also.

The pushup targets obvious muscle groups like chest, shoulders, triceps but also lower back, legs, hips, lats and abdominal muscles. Pushups are a great exercise for core strength and are considered by some the perfect body weight resistance exercise. Start your pushups plan today.  

Resistance training like pushups is good for men and women and can help build up bone density.  As long as you don't over do it for your age and fitness level you can expect to see gradual and encouraging improvement with regular practice.  Start small, be consistent, protect your gains.