The heater on your car serves 2 purposes: to keep the engine cooled off and to keep the interior of the car warm. The air conditioning system is designed to keep the passenger compartment cool. The heater warms up the passenger area by extracting heat from the engine using your engine coolant and carrying it through the heater core, which then runs into the passenger compartment. The air conditioning system takes heat and moisture out of the passenger compartment using a refrigerant called either R12 (in older vehicles) or R134A (in newer vehicles). Both systems have several components and should only be worked on by qualified professionals.