The energy stored in food is measured in terms of calories.
Technically, 1 calorie is the amount of energy required to raise the temperature of 1 gram of water 1 degree centigrade. The calorie measure used commonly to discuss the energy content of food is actually a kilocalorie or 1000 real calories. This is the amount of energy required to raise 1 kilogram of water (about 2.2 pounds) 1 degree centigrade.
Different foods contain different amounts of energy -- which is why a small piece of chocolate can have many more calories than a similarly sized piece of lettuce.
However, since calories are a measure of energy, there cannot be, as some diet books claim, different types of calories. A fat calorie has the same amount of energy as a protein or carbohydrate calorie.
A person's caloric need is determined using a variety of mathematical equations. Age, height, current weight, and desired weight are taken into account. Diet is what you eat. Dieting usually refers to eating fewer calories to lose weight.