It is primarily a cardiovascular exercise, which means it benefits the heart and circulatory system and burns calories. If you expend more calories than you consume in a day, you lose weight. If you expend an equal amount of calories, you maintain your weight, as the women in my class were.

Resistance training or weight training is paramount in changing the shape of your body. Beyond burning calories, lifting weights adds muscle and changes your body's metabolic makeup.

For example, let's say you had a genetic twin who was the same height and weight. But your twin possessed a higher amount of muscle mass, and therefore a higher RMR. That twin would burn more calories walking, watching television or even sleeping simply because her body consumes more calories to function.