The terms physical activity and exercise may seem synonymous, but there are critical differences between the two.

Levels of physical activity can be viewed on a continuum. At one end of the spectrum is a sedentary lifestyle, an absence of any significant physical activity. On the other end is the peak level of exercise training exhibited by a well-conditioned athlete. How you move along this continuum depends on both your starting point and your exercise goals. If you are currently sedentary, simply upping the amount of physical activity in your daily life is an important push forward.

At a certain point, however, you will need to establish an exercise program to continue making strides. And once you have a program in place, you should periodically review your fitness level and reexamine your personal goals to determine how you can continue to make progress.

When you call on your body to perform any type of physical work — even something as simple as walking across the room — your cells are jolted out of their resting state of balance. What follows is a complicated set of physical processes that supply the cells with the extra energy they need.

First and foremost, the cells require additional oxygen to fuel the metabolic functions necessary for sustained activity. To meet this demand, the respiratory and circulatory systems gear up to deliver oxygen-rich blood to the working muscles. This accelerated metabolism, in turn, produces greater amounts of waste products. These products are transported back to waste-removal sites, such as the lungs and kidneys, for expulsion from the body.

It is because of these processes that you breathe more rapidly and your heart pumps more vigorously when you exert yourself. Another byproduct of increased energy production is heat. Sweating — a telltale sign of strenuous exertion — is your body's way of cooling your skin and keeping your body temperature at a safe level.

Physical activity (and exercise) is a cornerstone of a healthy lifestyle. Not only does physical activity make you look and feel better, but it is also critical for improving your health and extending your life. Being active significantly lowers your chances of developing potentially fatal illnesses, including heart disease, diabetes and cancer. In addition, remaining active throughout your life can help you stay healthy and disability-free as you age.

Here's a rundown on the benefits you can expect from regular activity: