Physical fitness is most easily understood by examining its components, or "parts." There is widespread agreement that these four components are basic.
Cardiorespiratory Endurance - the ability to deliver oxygen and nutrients to tissues, and to remove wastes, over sustained periods of time. Long runs and swims are among the methods employed in measuring this component.
Muscular Strength - the ability of a muscle to exert force for a brief period of time. Upper-body strength, for example, can be measured by various weight-lifting exercises.
Muscular Endurance - the ability of a muscle, or a group of muscles, to sustain repeated contractions or to continue applying force against a fixed object. Pushups are often used to test endurance of arm and shoulder muscles.
Flexibility - the ability to move joints and use muscles through their full range of motion. The sit-and-reach test is a good measure of flexibility of the lower back and backs of the upper legs.
Body Composition is often considered a component of fitness. It refers to the makeup of the body in terms of lean mass (muscle, bone, vital tissue and organs) and fat mass. An optimal ratio of fat to lean mass is an indication of fitness, and the right types of exercise will help you decrease body fat and increase or maintain muscle mass.
A MATTER OF PRINCIPLE
The keys to selecting the right kinds of exercises for developing and maintaining each of the basic components of fitness are found in these principles:
Specificity - pick the right kind of activities to affect each component. Strength training results in specific strength changes. Also, train for the specific activity you're interested in. For example, optimal swimming performance is best achieved when the muscles involved in swimming are trained for the movements required. It does not necessarily follow that a good runner is a good swimmer.
Overload - work hard enough, at levels that are vigorous and long enough to overload your body above its resting level, to bring about improvement.
Regularity - you can't hoard physical fitness. At least three balanced workouts a week are necessary to maintain a desirable level of fitness.
Progression - increase the intensity, frequency and/or duration of activity over periods of time in order to improve.
Some activities can be used to fulfill more than one of your basic exercise requirements. For example, in addition to increasing cardiorespiratory endurance, running builds muscular endurance in the legs, and swimming develops the arm, shoulder and chest muscles. If you select the proper activities, it is possible to fit parts of your muscular endurance workout into your cardiorespiratory workout and save time.
From: http://www.hoptechno.com/book11.htm