A calorie is a measure of energy of heat. The amount of heat that is needed to raise the temperature of one gram of water one degree centigrade equals one calorie. The caloric content in food is measured by how much heat that food can produce when burned. The food we eat is converted into glycogen and stored into our muscles to prepare for burning during exercise or any activity.
In short, calories are what your body burns as fuel. They are the source of energy that your body uses to power your heart, expand your lungs, press that 100 lbs. on the bench press, get you through that 40 minute aerobic workout; calories are what your body uses for every single function you do through out the day.
Everything you eat is made up calories. When you eat your body breaks down the nutrients in the food into glucose. If your body doesn't need the energy right away, the calories are converted into fat and stored for later use.
There are 3,500 calories in a pound of fat. So every 3,500 calories that you do not utilize turns into one pound of fat. So the importance of exercise and staying active is of great importance.
Remember that one fat gram is equal to 9 calories. One protein and carbohydrate gram is equal to 4 calories. Click here to find out how to keep track of those calories that you are consuming.