Article From WiseGeek:
To understand what a car radiator does, it might help to understand the nature of the internal combustion engine it protects. A car's engine has numerous moving parts, and where there is movement there is friction. Friction creates heat. Motor oil is pumped throughout the engine block to provide some lubrication, but it isn't enough to overcome all of this excess heat energy. As a result, parts of the engine become boiling hot as part of normal operations.
This is where the radiator system comes into play. The engine block must be kept relatively cool to avoid serious problems like overheating and seizure. If the pistons cannot slide freely in their cylinders due to excessive friction, they will eventually snap and cause total engine failure. To prevent this from happening, a mixture of water and anti-freeze is pumped through chambers in the engine block to absorb the excess heat and draw it away from vital areas.
When this superheated engine coolant exits the engine block, it returns to the radiator through a large rubber hose. A car's radiator is designed to maximize surface area through a significant number of internal folds and chambers. As the hot engine coolant moves through these nooks and crannies, excess heat is drawn out through the walls of the radiator. An electrical or belt-driven fan may force cooler outside air through the radiator to accelerate this cooling process. As the car moves, the front of the radiator is also cooled by the outside air coming through the car's grill.
By the time the superheated engine coolant has made its way through all of the chambers of the radiator, it should be cool enough to make a return trip through the engine block. However, if the coolant flow should be reduced by a blockage or loss of fluid, the engine block will not be cooled down and the remaining engine coolant will boil over. This is why maintaining a full coolant level is so important, especially during hot weather or long drives.
A radiator does not contain any electronic parts of its own -- special sensors register the temperature of the coolant as it exits the radiator. Engine coolant does not have to be especially cool in order to be effective, so there is usually a wide range of temperatures considered to be within normal parameters. If anything goes wrong with the radiator itself, such as a leak or broken hose, the operating temperature of the car can reach a dangerous level within minutes. The engine must be allowed to cool down naturally before the vehicle can be driven safely to a mechanic.