A calorie (cal) is a unit of measurement that indicates the amount of energy we obtain from a particular food. It is a unit of thermal energy equivalent to about 4.185 J.
By definition, one calorie is the energy needed to increase the temperature of 1 g of water by 1 °C at 15 °C under normal atmospheric pressure (76 cmHg). In nutrition, it is used to quantify the energy of food and the defining quantity of water is 1 kilogram, making the "food calorie" 1000 times as large as the other calorie. Hence, 1 calorie (nutrition) = 1 kcal = 1000 cal, and it is also called the large calorie.
The calorie is not an SI unit where the joule is the only unit of energy. The kilocalorie is frequently used in chemistry because it is a convenient measure of molar free energy: for example, 1.4 kcal/mol engenders a change in equilibrium by a factor of 10.