The radiator is a cooling device that consists of a series of pipes that have thin metal fins that can disperse heat quickly. The coolant in an engine gets really hot and the water pump is responsible for circulating the coolant through the engine and the radiator. The coolant is the hottest as it enters the radiator at the top and as it travels through the radiator it is cooled down by the time it re-enters the engine via the lower rad hose. The air flow from a moving vehicle and the cooling force provided from the rad fan is what helps keep the coolant cool and what helps regulate that temperature. The rad commonly houses a switch that senses the temperature of the coolant inside the radiator and turns on and off the fan accordingly. The reason that a radiator has to be changed is normally due to damage or corrosion. If the metal deteriorates to the point that perforation is caused, then the ensuing leak is a reason for replacing. A simple stone to the right spot in the radiator could be a cause of a leak as well. In more rare instances one might experience a clogged radiator, but that is likely due to a much larger problem because the only thing flowing through the radiator should be engine coolant.