The energy stored in food is measured in terms of "calories". Different foods can be used by the body to produce different amounts of energy -- which is why a small piece of chocolate can have hundreds more calories than a similarly sized piece of lettuce. The amount of calories in a diet refers to how much energy the diet can provide for the body. A well-balanced diet is one that delivers an adequate amount of calories while providing the maximum amount of nutrients. A person's caloric need is determined using a variety of mathematical equations. Age, height, current weight, desired weight and height are taken into account. The body breaks down food molecules to release the energy stored within them. This energy is needed for vital functions like movement, thought, growth -- anything you do requires the use of fuel. The body stores energy it does not need in the form of fat cells for future use. Weight gain or loss is the result of increased intake of food, decreased activity or both and vice-versa.
Technically, a calorie is how much heat is required to raise one gram of water one degree Celsius at the pressure of one atmosphere. In theory, approximately 3500 calories equals one pound of body weight.