It's an amount of energy. calorie (lowercase c) is the amount of energy needed to raise the temp of 1 gram of water by 1 degree Celsius.
A Calorie (uppercase C) is really 1,000 calories and is usually used to measure the energy available in food. The energy is stored in the bonds of food, and so typically, the more bonds your body can break, the more Calories contained in the food. We approximate how much energy is within the food using Calories.
