How are calories used and stored?

Calorie (upper case C) refers to food calories, or kilocalories. It's used to measure the amount of energy foods or individual nutrients provide when consumed.

calorie (lower case c) refers to energy in general, not food energy. It's used to measure the amount of energy needed to increase the temperature of 1 gram of water by 1 °C at standard atmospheric pressure (101.325 kPa)