Your body needs a certain amount of energy to maintain basic functions such as breathing, blood circulation and digestion. The energy required to keep your organs functioning is referred to as the resting or basal metabolic rate.
Any time you are active, additional energy is required. It is obtained from glycogen (carbohydrates) and fat stored in the blood, liver, and muscles. The key to losing weight is to draw on the fat rather than on the carbohydrate reserves.
Which of the two energy sources you use depends on the intensity and duration of your activity. The higher the intensity, the more your body will pull from the stored carbohydrates. The lower the intensity, the more your body will rely on fat as its fuel.
Aerobic exercise (any activity that increases oxygen intake and heart rate) is most effective for weight loss. When you perform aerobic activities you repeatedly contract large muscle groups such as your legs and arms. Walking, running, rollerblading, swimming, dancing, and jumping jacks are all forms of aerobic activity. Surprisingly, if your aerobic activity is low to moderately intense and of long duration, you will burn more fat than if you had participated in a short burst of high-intensity exercise. In short, a brisk 30-minute walk will burn fat while a 100-yard sprint will burn glycogen.