A kilocalorie is the amount of energy needed to heat a liter of water 1 degree Celsius, while a calorie is the amount of energy needed to heat a milliliter of water the same amount. The dietary calorie, used to measure energy in food, is identical to the standard kilocalorie.
The nutritional labels on food that list calories are actually listing the food's energy content in kilocalories. Scientists use the abbreviation "kcal" to avoid confusion between standard calories and dietary calories. Sometimes, capitalization is also used to distinguish between the two, with the dietary Calorie being treated as a proper noun.