The radiator works as part of the overall cooling system and so relies on a number of other parts to function correctly. Some of the more pertinent parts are the upper and lower radiator hoses, the radiator cap and the radiator overflow tank, sometimes called the coolant overflow bottle, expansion tank or reservoir.
The radiator cap is more complicated than it may appear at first glance. It might seem that it simply caps off the radiator, allowing you to add coolant if necessary. It also maintains the proper pressure of the radiator, though. Inside the cap is a spring mounted plunger that controls the pressure inside the radiator. Keeping a high pressure inside the radiator actually increases the boiling temperature of the coolant. If the pressure becomes too high, though, the plunger moves and allows some coolant into the overflow tank. When the coolant cools it condenses, creating a vacuum that pulls coolant back in from the overflow tank.