A calorie* is a unit of energy that is used to measure the amount of energy stored in food. So one food calorie is the amount of energy (heat) it takes to raise the temperature of 1 kilogram of water 1.8°F (1°C).
The Calories you see on food labels are actually kilocalories, and are also called food calories. One kilocalorie is equal to 1,000 calories. This is why food calories are listed as “Calories” with a capital C. One Calorie equal 4,184 joules of energy.