This article is from the Beer and Other Brewing FAQ, by Kurt Swanson email@example.com with numerous contributions by others.
A wort chiller is a device used to quickly cool boiling wort to
yeast pitching temperatures. Two common constructions are the
immersion chiller and the counterflow chiller. The immersion
chiller consists of a coil of copper tubing that is immersed in
the wort, and cold water is run through the tubing. Counterflow
designs usually consist of copper tubing inside of a larger diameter
plastic tubing; cold water runs through the plastic tubing in one
direction, cooling wort runs through the copper tubing in the other
Using a chiller to quickly cool wort has several advantages over
slow air cooling. You get your yeast pitched quickly, reducing
the risk of infection; the time the wort spends at DMS* producing
temperatures is reduced; and a quick chill promotes good cold break.
* DMS is Dimethyl Sulfide, a malt by-product with an aroma
described as similar to cooked corn.