A calorie is a unit of energy. Specifically, it is the amount of energy that is required to raise the temperature of one millilitre/ml (which is also one gram) of water by one degree Celsius. During the late 19th Century, scientists started to be interested in the amount of calories in different foods. Using a device called a bomb calorimeter, a known amount of food which has had its water content evaporated was placed in a container surrounded by a known amount of water. The container was sealed, oxygen piped in, and the food ignited. From the rise in temperature of the water, the calorie content of the food was calculated. Using this device, a chemist named Wilbur O. Atwater determined the calories in 1 gram of fat, carbohydrates, and protein (9-4-4), and consequently calculating the total calories of a specific food based on its content of fats, carbohydrates and proteins.
When referring to the energy content of foods, we talk about Kcal (kilocalories) – the energy required to warm up a kilogram of water by 1 degree Celsius.