When it comes to weight loss, the single most important thing you must keep in mind is the energy balance. In fact, this point is so important hundreds of books have been written about it. It's the funding principle of every single weight loss program and it explains why you lose or gain weight.
The energy balance is the difference between your intake and output of energy.
IN - OUT = BALANCE
Your IN is the food you eat. Food contains energy (that's why you die if you starve for weeks). The more you eat, the bigger your IN.
To keep things simple, your OUT is the energy you spend in physical activities (it's a little more complicated than that, but we'll keep things simple here. Keep reading). Physical activity doesn't mean just sports or exercise. Walking from home to work, walking around the shopping mall or grocery store all count. Even washing the dishes does, to a certain extend. We spend energy all day long. Anytime you're not seated or lying, you spend more than the minimum. Of course, the harder the activity you're doing, the more energy you spend.
At the end of the day, the BALANCE can be either:
1) Positive. More energy IN from food than OUT from physical activities. That extra energy is stocked by your body as fat for later use. The problem is, when the balance stays positive for a long period of time, you store more and more fat and become overweight.
2) Negative. Good news. More energy OUT from physical activities than IN from food. The missing energy is taken from your energy store-yep, fat tissue. If the balance stays negative for a long period of time, you lose some of that tissue and become leaner.
3) Neutral. Your IN equals your OUT. No change in energy (fat) stores, no change in body weight.