A cars cooling system carries heat away from the engine. This is a very important duty, as engines will quickly overheat and result in catastrophic damage if components are broken or malfunctioning. The main components of the cooling system include the thermostat, water pump, radiator, and expansion tank. The expansion tank holds extra coolant (the fluid inside the cooling system) and contains pressure. The radiator is like a system of vents in the front of the car. Air is able to pass through the vents and cool down the coolant flowing through the radiator. The water pump is the pump driven off of the main drive belt, and it is what keeps the coolant flowing within the engine/system. The thermostat regulates how much coolant returns into the engine block and how much goes back through the radiator.
My car has 140,000 miles, and I replaced the following items:
I did this as a preventative measure so I won't be stuck on the side of the road in the future. I also replaced some other accessories in the cooling system including some sensors and hoses. With the cars age, mileage, and engine heat cycles, lots of plastic components were very brittle and the parts were all original, so I felt that it was a smart choice replacing all the components in one go.
Here's a basic diagram of a cars cooling system
