KEG most often used package in the world for beer made from stainless steel or aluminum. In past was this type of package made from wood and used for storage nails, gunpowder and a variety of liquids. In this age it is very popular for storage beer, sodas, cider or wine and its transport. Such a beverages are generally kept in the keg under pressure. 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

In the world are used several types of KEGs divided depending on country and its common system.

• U.S. keg sizes (1⁄2 barrels of 15.5 gallons,  1⁄4 barrels of 7.75 gallons, and  1⁄6 barrels of 5.17 gallons)
• DIN keg and EURO keg (same volumes 20,25,30 and 50 liters but different dimensions – diameter/height)

Special types of KEGs

• Cornelius kegs
• Mini kegs 5 L
• Beer balls
• One way kegs (Petainers or Key-Kegs)