A wort chiller is used to reduce the temperature of boiled wort to a temperature in which yeast can be safely pitched. This means that the wort needs to be below 80 degrees F to insure that the yeast is not killed at pitching.
Wort is most vulnerable to contamination from wild yeast and bacteria when the temperature is below 130 degrees F, and not yet cooled enough in which yeast can be added. When the wort is cooled faster, the yeast can be pitched sooner reducing the risk.
Rapid cooling of the wort helps to facilitate a cold break. A cold break is the coagulation and precipitation of protein and tannin. This begins to occur when the wort temperature drops below 140 degrees F and increases as the temperature drops. The result will be a clearer beer.
Wort chillers come in two styles: the Counter-Flow and the Immersion. An Immersion chiller is a copper coil submerged in the boiled wort through which cold water flows. A Counter-Flow chiller has copper tubing inside a larger hose. The hot wort flows through the copper tubing while cold water runs through the larger hose in the opposite direction. The cold break using an Immersion chiller will be in the boiling vessel while it will be in the receiving vessel using a Counter-Flow chiller. Counter-Flow chillers use less water and cool more quickly than Immersion chillers. The length of the copper tubing and the temperature of the ground water will influence the effectiveness of ether style.