the engine drives the water pump and the pump pushes cool water around the engine. this cools the engine. at the same time, the water becomes hot. the water in a hot engine is normally around 110c. the hot water then passes through the thermostat. this controls the temperature of the engine. from the thermostat, it flows through the top hose into the radiator.
here, a fan cools the water, and the cool water sinks to the bottom of the radiator. the cool water then leaves the radiator. it flows along the bottom hose, passes through the pump and enters the engine again.