Vapour compression is the principle by which 12-volt refrigeration systems work. The process requires a liquid refrigerant to change its state to a gas and then back to liquid inside of a series of sealed pipes. This change absorbs energy in the form of heat.
A series of tubes on the outside of the refrigerator, called the condenser, are connected to a series of tubes on the inside, called the evaporator. One end of the condenser is connected to the evaporator with an expansion valve that restricts flow, while the other end connects to a compressor that pressurizes the system.
The refrigeration cycle starts with the liquid, under high pressure, being forced through the expansion valve. This allows the liquid to expand, under lower pressure, into a gas inside of the evaporator. As the gas passes through the metal tubes, it fully vaporizes, drawing heat from the metal, which in turn draws heat from the refrigerator compartment. Once the refrigerant has been vaporized, it is sucked into the compressor and forced back into the condenser at a much higher pressure. A fan pulls ambient air over the condenser, drawing heat and allowing the vapor to return to liquid form. The refrigeration system does not create cold air; it simply transfers heat.