what's a "fan clutch"?

okay, so the part i fixed on the van turns out has NOTHING to do with the problem...weird.
it's the "fan clutch".

this part of the car hooks up to the fan and some sort of something happens. well the fan clutch went out today while i was in town, and now the fan thinks it needs to be cooling things off all the time because it went out.
our mechanic is going to search around town for the part tomorrow.
i just love cars.