Calorie is a unit of energy that can be defined in two broad categories.
The small calorie, or gram calorie (cal), is the amount of heat energy needed to raise the temperature of one gram of water by one degree Celsius at a pressure of one atmosphere.
The large calorie or kilogram calorie (cal or kcal), also known as the food calorie, is the heat energy required to raise the temperature of one kilogram (rather than a gram) of water by one degree Celsius. It is equal to 1,000 small calories.
In nutrition and fitness calorie is often used as a measurement of the amount of energy that food provides.