One thing I learned from the nutrition class I just finished was that the majority of calories we burn is from basic living - breathing, sitting, thinking, eating, sleeping, etc. This is called your basal metabolic rate, what many people simply call your metabolism. Although exercise doesn't burn up the majority of calories, it can rev up our metabolism, so even after we've finished exercising, the rate at which our bodies burn calories increases. In addition, I learned that we burn three times more calories standing instead of sitting. (Read about it on Livestrong.)
Both the lack of exercise in the general population and the increased tendency to sit slows our metabolism which can lead to weight gain. Think about how modern humans in the Western world spend their days. Most of us get in our cars to drive to work, sit at a desk or at some type of station all day, sit in our cars to return home, then sit on the sofa to watch our favorite TV shows because we feel too tired to exercise. Not a good combination.
I encourage readers to find ways to get on your feet. Even better - try to squeeze in some time to exercise, even if it's just walking around the block during your lunch hour or before dinner when you get home. If you have children, make it a family event as children are also far too sedentary.