Sparging, also called lautering is a step at the end of the mashing process where hot water is run through the grain bed to extract a sweet liquid called wort. The wort is later boiled and fermented to produce beer.
The Sparging Process
After the mashing process is complete, the grains, water and sugar are still in suspension in the mash container, called the mash tun. The sugars are separated from the grains in a process called sparging. The mash tun typically has a false bottom or screen at the bottom with a spigot that allows the brewer to draw run-off from the bottom of the grain bed. Hot water at approximately 178 F is slowly added to the top of the grain bed, run through the bed, and drawn off the bottom through the false bottom and out the spigot to the boiling vessel. This extracts sugars from the grains and produces sweet liquid called wort for boiling. The initial runnings (first few quarts) drawn during the sparge process are recirculated back through the grain bed, as the early runnings often contain grain husks, crushed material and other undesirable elements. After the initial runnings, the grain bed will act as a filter and reduce the cloudiness of the runnings. Sparging is best done slowly so that a maximum amount of sugar can be extracted from the spent grains. The sparged wort is transferred to a boiler where hops is added and the mixture boiled before cooling for fermentation.
(Editors Note - I've marked this as a stub. This article should detail the different types of sparging (fly sparging, batch sparging, stuck mashes, etc...)