Why do we need a fan!!?, Here’s the answer The RADIATOR needs a constant flow of air through its core to cool it adequately. However it happens when the car is in motion but when it’s stationary a fan is used to help the airflow which would be driven by engine power. When the engine isn’t working hard there’s no need of turning it on. So switching off and on would be done by thermostat ( temperature sensor ) whenever needed. 

 Usually Two fans cover more Radiator surface area than one large fan, which makes this twin-fan system more efficient. One more thing to know is straight blade fans move more air than the curved-blade fans do. 

Water pump: It helps in driving the coolant through the cooling system from the Radiator to the engine and back to radiator as long as the engine is working. 

The water pump is a centrifugal pump. The pump has a shaft with a pulley on one end and a pump motor on other end. When the pulley is spun by a belt the rotor moves the coolant.

Water pump is driven by either of the drive belt or time belt.

Drive belt: ( fan, serpentine ) is used to provide power and drive to the necessary components to make car work such as air-conditioning, alternator, water pump, power steering etc. 

Time belt: Also known as camshaft belt, connects engine cylinder head to engine cylinder block and moves the inner engine components in synchronized timing to ensure correct compression and combustion cycles that make an engine run. However this belt is always behind a cover and is not meant to be service often.

Most modern cars uses time belt to drive water pump.