A calorie is a measure of energy. Calories are the energy that fuels our body, similar to the way gasoline fuels our car.
One calorie of energy is the amount of heat energy needed to raise the temperature of one gram of water by one degree Celsius. One calorie from fat has the same amount of energy as one calorie from protein or one calorie from carbohydrate. To determine the number of calories (or energy) provided by a certain food, the food is placed in a sealed container and burned. The amount of heat generated indicates the caloric content of the food.
Understanding the amount of calories needed by your each day and how that compares with the amount of calories you are taking in is very important in meeting your personal health goals.