Why does the radiator fan come on and does it only come on when the car is running hot?

Why does the radiator fan come on and does it only come on when the car is running hot?
An automatic radiator fan will go on and off in cycles to cool an engine down when it begins to heat up. It works with sensors that monitor the engine heat and trigger other sensors to turn on the fan. It will even run with the car engine off after it's been driven a long time and is hot enough to need it. Then it will turn off on it's own. It's normal operation. IT's when the fan doesn't come on when the engine is hot that you need to worry, or when your temp gauge is showing hot and you check for the fan to see if it is turning on and off and it's not working that you have a problem.