Calories (thermoc.)
A thermochemical calorie (cal_th) is defined as exactly 4.184 joules. It is roughly the energy needed to warm one gram of water by one degree Celsius.
Calories (I.T.)
The Calorie (I.T.) or International Table Calorie is defined as the amount of energy required to raise the temperature of one gram of water by one degree Celsius at a pressure of one atmosphere which is approximately 4.1868 joules.