Upon making exercise recommendations to clients, friends and loved ones, I’m frequently met with the response . . . “I get plenty of exercise . . .I walk my dog every day, I play golf and walk the course, I park in the spot furthest from the store, I walk up the stairs rather than taking the elevator, I have small children and am constantly chasing after them”.  All of this is great and means that their lifestyle is not completely sedentary but if the goal is to improve fitness, it is a mincing of words to call these physical activities exercise.
Physical activity is defined as any activity that causes your body to work harder than normal.  Exercise is a form of physical activity, however, the intent is markedly different.  The dictionary defines exercise as bodily exertion for the sake of developing and maintaining physical fitness.  This, of course, begs the question, what is physical fitness?  Unfortunately, the ‘definition’ of physical fitness is a bit more allusive as it means different things to different people.  For you, it might be a certain weight or clothing size.  For your physician it might be your blood pressure or cholesterol.  There are plenty of very physically fit people who are heavier than they’d like to be and plenty of thin people who are extremely unfit.
In the interest of clearing up some of the confusion surrounding what is exercise and what is physical activity, I would liken physical fit body to the state of a fine-tuned, high performance automobile.  It can be big or small but regardless, it requires high octane fuel, it needs to be driven fast, and it requires regular maintenance to ensure continued performance.  This high performance vehicle is not meant to sit in the garage for months on end, it needs to be driven – around town at slow speeds is better than nothing but it will function better if you push it to the limits regularly.  In terms of the human body, the high octane fuel would be high quality nutrients provided at regular intervals.  The maintenance would be annual visits to your health care provider to ensure all systems are go.  The basic guidelines to the ‘driving’, or exercise, the body requires: consistency, duration, intensity and variety.  These guidelines can be used as a litmus test for whether you are engaging in exercise that will improve or maintain physical fitness or if you are simply leading an non-sedentary lifestyle.
If you are looking for more concrete guidelines, view the link to the 8/1/07 physical activity guidelines jointly put forth by the American College of Sports Medicine and the American Heart Association.