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. Any activity that raises your heart rate and keeps it up for an extended period of time will improve your aerobic conditioning.
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). Weight training (resistance training) or simple exercises such as push-ups are two examples of ways to focus on muscle strengthening.
Flexibility. 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.
Understanding the differences between each kind of fitness will help you set your fitness goals. Reaching a balance between the three is important, because they affect each other and each contributes to total fitness.
Some physical activities involve more than one kind of fitness. Some activities that are thought of as aerobic exercise, for example, also strengthen muscles (swimming, cycling, skiing).