A barrel is one of several units of volume applied in various contexts; there are dry barrels, fluid barrels, oil barrels, and so forth. For historical reasons the volumes of some barrel units are roughly double the volumes of others; volumes in common use range approximately from 100 to 200 litres. In many connections the term drum is used almost interchangeably with barrel.
Ale casks at a brewery in the UK. These are firkins, each holding 9 imperial gallons (41 L) or a quarter of a UK beer barrel.
While the barrel as a unit of measurement for oil is 42 U.S. gallons, actual barrels used in industry are typically 55 U.S. gallons or 200 litres internationally.
A barrel or cask is a hollow cylindrical container with a bulging center, longer than it is wide. They are traditionally made of wooden staves and bound by wooden or metal hoops. The word vat is often used for large containers for liquids, usually alcoholic beverages; a small barrel or cask is known as a keg.
Traditional oak barrels made by Chilean cooperage TonelerĂa Nacional
Modern stainless steel casks and kegs outside the Castle Rock microbrewery in Nottingham, England
Wooden wine barrel at an exhibition in Croatia
Pyramidal pile of herring barrels in Scheveningen, the Netherlands, c. 1940