A stainless steel immersion chiller is a great piece of equipment for all-grain and extract brewers to cool the wort at the end of the boil. It is immersed into the boiling wort at least 10 minutes before the end of the boil (this will help to sanitise the chiller, although it should be clean before being used) and attached to a cold water source.
When ready to cool the wort, turn on the cold water and it will flow through the immersion chiller and cool the wort.
To increase the rate at which the wort cools you can stir the wort to create a whirlpool. This will cause more of the wort to come into contact with the chiller and cool the wort faster. Make sure to sanitise whatever you use to stir the wort.
You could also use a pump (such as the MKII 25w High Temp Magnetic Pump) to create a whirlpool immersion chiller; made popular by award winning American home brewer Jamil Zainasheff. Here is a link to Jamil's site, Mr Malty, with more details.
An immersion chiller can also be used to pre-chill the cooling water. Just submerge the immersion chiller in an ice bath to to pre-chill the water that ultimately flows into another immersion chiller in the wort. This can be particularly helpful during summer when the groundwater temperature increases.
This chiller comes complete with a garden hose fitting on one end to connect to your garden or kitchen tap, and clear vinyl tubing.