Radiator and Heater Cores
The radiator is the major player in the cooling system that cools the engine. Air being pulled through the radiator by the fan, as well as ram air from forward motion of the vehicle is what dissipates heat and cools things off. The thermostat regulates the coolant temperature by the amount of time it allows the coolant to flow through the radiator.
The heater core is basically a small radiator that works in reverse, in that the heater fan blows across the heater core to push warm air into the passenger compartment.
The radiator, heater core, and cooling system must be flushed on a regular basis to keep the cooling system free-flowing so the radiator can cool the system. Generally, cooling systems should be flushed at least every other year or about 30,000 mile intervals.