File:Beer_barrels_galore.jpg · Wikimedia Commons · See Wikimedia Commons
A barrel is a unit of measurement used to measure volume, but its size varies depending on what substance is being measured or what country is using it. This makes barrels useful for trade and commerce, though the lack of a single standard size means people need to specify which type of barrel they're referring to.
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Encyclopedic overview
Ale casks at a brewery in the UK. They are firkins, each holding 9 imperial gallons (41 L), or a quarter of a UK beer barrel.
A barrel is one of several units of volume applied in various contexts: there are dry barrels; fluid barrels, such as the British beer barrel and American beer barrel; oil barrels, etc. 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 (22 to 44 imp gal; 26 to 53 US gal). In many connections, the term drum is used almost interchangeably with barrel.
Excerpted from Wikipedia’s “barrel” article, available under the CC BY-SA 4.0 licence.