Well, what it is- is keeping the engine's temperature controlled.  So in essence, it is exactly as it sounds--- cooling system (system=engine).  So cooling the engine happens due to "coolant" and major components such as the radiator, radiator cap, coolant reservoir, fan, fan relay, water pump, hoses, belts, thermostat, coolant sensor and ANTIFREEZE.  The coolant from the engine is circulated by the water pump to the radiator to remove  waste heat.    The coolant usually travels and exits via the upper radiator hose and returns back through the lower radiator hose. 

Now, the actual temperature is controlled by the thermostat that has this (metal) flap that allows heat passage.  It sits where the upper radiator hose connects to the engine.  And yep, you guessed it... that is why we recommend removing the thermostat so by chance it doesn't open which overheats your engine.