Simple answer is a sensor (like the one you may be using for your temperature gauge) that goes to a relay which then turns the fan on/off. There are different temperatures available (including some that are variable but these tend to be more expensive) and some that even will turns the fan on slowly and increase the speed as the coolant gets hotter.
They also sence the heat in different ways - some use a probe in the fins of the radiator, some go in the cooling system (like the temperature sensor for your gauge) I saw one that went into the upper radiator hose.
Lots of good sources - Summit or Jegs may be the easiest. Here are some examples:
