Keg
A keg is a small barrel.
Traditionally, a wooden keg is made by a cooper and used to transport items such as nails, gunpowder, and a variety of liquids.
More recently, a keg is often constructed of aluminum or steel. It is commonly used to store, transport, and serve beer. Other alcoholic or non-alcoholic drinks, carbonated or non-carbonated, may be housed in a keg as well. Such liquids are generally kept under pressure.
Beer kegs are made of stainless steel, or less commonly, of aluminium. A keg has a single opening on one end, called a "bung." A tube called a "spear" extends from the opening to the other end. There is a self-closing valve that is opened by the coupling fitting which is attached when the keg is tapped. There is also an opening at the top of the spear that allows gas (usually carbon dioxide) to drive the beer out of the keg. The coupling fitting has one or two valves that control the flow of beer out of and gas into the keg. The keg must be in the upright position, that is, with the opening on top for the beer to be dispensed. Kegs can be contrasted to casks, which have two or more openings and no spear. Most major breweries now use internally speared kegs.
Historically a beer barrel was a standard size of 50 gallons, as opposed to a wine barrel at 32 gallons, or an oil barrel at 42 gallons. Over the years barrel sizes have evolved and breweries throughout the world use different sized containers. Even when the content capacity of two kegs are equal—e.g. metricized to 50 liters—the keg shape and keg tap system may differ greatly.
Most U.S. brewers sell beer in 1⁄2 barrels of 15.5 gallons, 1⁄4 barrels of 7.75 gallons, and 1⁄6 barrels of 5.17 gallons.
...
Wikipedia