As Bob explained, the coolant temp controls when the fan should kick on. There is, however, a relay involved. It's one of the two relays mounted on the front side of the RH strut tower. (The other one, known as the radio suppresion relay, feeds power to the ignition/fuel system.) They look the same, and yes, they're interchangeable.

It's only supposed to run at extremely hot engine temps. When they do run, it's usually only a minute or two. The power goes to the temp sensor on the radiator. When it gets hot, the switch closes and the fan starts. Most folks wire them to run with the A/C. Just remember, the A/C runs with defrost and may overcool in winter.