Bringing down the temperature of the wort quickly is essential to brewing. Lowering the temperature quickly reduces chances of contaminating your beer with wild yeasts and bacteria, and will reduce the levels of DMS that can ruin a beer's aroma. A quick chilling of the wort will also improve clarity.
To use the immersion chiller, place it in to the brew pot in the last 15 minutes of the boil. This will sanitize it and kill any wild yeasts or unsanitary pests that might be introduced. Once the brew pot has been taken off the boil, you can begin cooling immediately by hooking up the line and turning on the water.
The large size is 50' long with 1/2" tubing. Fits really well in 10-30 gallon kettles.