In automobiles with an internal combustion engine, a radiator is connected to channels running through the engine and cylinder head, through which is pumped a liquid. This liquid is typically a mixture of water with ethylene glycol (a.k.a. antifreeze).
The fluid moves in a closed system from the radiator to the engine, where it conducts heat away from the engine parts. It then flows through a thermostat (controlling the rate of flow) back to the radiator, where it is cooled again by convection with the air.
This process cools the entire engine. Some engines have an additional oil cooler; a separate small radiator to cool the motor oil. Most modern cars use aluminum radiators. These radiators are made by brazing thin aluminum fins to flattened aluminum tubes.
The radiator cap increases the boiling point of the coolant by about 45 F (25 C). The cap is actually a pressure release valve which is usually set to 15 psi. The boiling point of water increases when water is placed under pressure.
After the fluid in the cooling system heats up, it expands, resulting in a build up of pressure. The radiator cap is the only place where the pressure can be released in the cooling system. When the pressure reaches about 15 psi, the pressure pushes the valve open and permits coolant to be released from the cooling system. This coolant flows through an overflow tube into the bottom of an overflow tank. This setup keeps air out of the cooling system. When the radiator cools back down, a vacuum is created in the cooling system that pulls open another spring loaded valve, bringing water back in from the bottom of the overflow tank to replace the water that was expelled.
The thermostat's allows the engine to heat up quickly and to keep the engine at a constant temperature. It does this by regulating the amount of water that goes through the radiator. At lower temperatures, the opening to the radiator is completely closed and all of the coolant is cycled through the engine.