As bruno and fatcock point out, you want to cool your wort as soon as possible. You could also go with an immersion chiller. They are easier to make and clean, but don't cool the wort quite as quickly. Mine cools a batch down from boiling in about 15 minutes, to about 70. The main reason is to reduce the risk of contamination. A slow cooling method will probably produce off-flavors, such as DMS (butterscotch taste). Also, cooling quickly will allow the haze causing proteins to fall to the trub layer. Hop aroma is also preserved more with rapid cooling.