A Radiator is a type of heat exchanger.  It is designed to transfer heat from the hot coolant that flows through it to the air blown through it by the fan.  Most cars today use aluminum radiators.  The coolant flows from the inlet to the outlet through many tubes mounted in a parallel arrangement.  The fins conduct the heat from the tubes and transfer it to the air flowing through the radiator.  By creating turbulence inside the tube, all the fluid mixes together, keeping the temperature of the fluid touching the tubes up so that more heat can be extracted, and all of the fluid inside the tube is used effectively.

Cooling system tank