Physical fitness is a state of health and well-being and, more specifically, the ability to perform aspects of sports, occupations and daily activities. Physical fitness is generally achieved through proper nutrition, moderate-vigorous physical exercise, and sufficient rest.
Discussion on Aerobic fitness. Aerobic activities condition your heart and lungs. Aerobic means "with oxygen." The purpose of aerobic conditioning is to increase the amount of oxygen that is delivered to your muscles, which allows them to work longer.
Discussion on Muscle strengthening. Stronger muscles can mean either more powerful muscles that can do bigger jobs (such as lifting heavier weights) or muscles that will work longer before becoming exhausted (endurance).
Like aerobic fitness and muscle strengthening, flexibility is a result of physical activity. Flexibility comes from stretching. Your muscles are repeatedly shortened when they are used, especially when exercising. They need to be slowly and regularly stretched to counteract the repeated shortening that happens through other activities.