Heating & Cooling
Just like in your home your automobiles HVAC provides heating and cooling to make driving pleasant.
Heating works by using the the cooling system of the engine through the process of heat exchange to push hot coolant through a heater core which is like a little radiator to provide heat to the interior cabin. A blower system pushes air across the heater core fins to transfer that heat to the cabin air. In the cabin there are heater box doors which change the direction of the interior flaps to provide defrost, face and foot well air flow. This is traditionally done by a manual cable but on most modern cars is done electrically.
Failures can occur when coolant is lost, engine temperature is too low, maintenance on the cooling system is incomplete and a plugged heater core occurs or the interior distribution system fails.
Air conditioning works in the same manner as heating internally a flap closes of the heat section or a valve shuts off coolant flow. Distribution is provided by the same system as heating. Externally air conditioning uses a pressurized piping system that changes the temperature of a gas to provide cooling. The exchange occurs multiple times in the system with the final result in the Heater/Air conditioning box. As with the heater core the blower system blows air across the evaporator core pushing cold air through the distribution system.
The most common failures on ac systems is a leak, if quantity drops there can be no exchange of pressure and no ac.
On most later model cars there is also a cabin air filter to provide clean air to the interior. This must be changed on a regular basis to prevent clogging and reduced air flow, just like your home heater system.