There are three kinds of fitness:

There are three kinds of fitness:

• 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).