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.

Modern wooden barrels for wine-making are made of French common oak (Quercus robur), white oak (Quercus petraea), American white oak (Quercus alba), more exotic is Mizunara Oak all typically have standard sizes: Recently Oregon Oak (Quercus Garryana) has been used.

• "Bordeaux type" 225 litres (59 US gal; 49 imp gal),
• "Burgundy type" 228 litres (60 US gal; 50 imp gal) and
• "Cognac type" 300 litres (79 US gal; 66 imp gal).

Modern barrels and casks can also be made of aluminum, stainless steel, and different types of plastic, such as HDPE.

Someone who makes barrels is called a "barrel maker" or cooper (coopers also make buckets, vats, tubs, butter churns, hogsheads, firkins, kegs, kilderkins, tierces, rundlets, puncheons, pipes, tuns, butts, pins, troughs and breakers).

Barrels have a variety of uses, including storage of liquids such as water, oil, and alcohol. They are also employed to hold maturing beverages such as wine, cognac, armagnac, sherry, port, whiskey, beer, arrack, and sake. Other commodities once stored in wooden casks include gunpowder, meat, fish, paint, honey, nails and tallow.

Early casks were bound with wooden hoops and in the 19th century these were gradually replaced by metal hoops that were stronger, more durable and took up less space. The term barrel can also refer to roughly cylindrical containers or drums made of modern materials like plastic, steel or aluminium.

The barrel has also been used as a standard size of measure referring to a set capacity or weight of a given commodity. For example, in the UK a barrel of beer refers to a quantity of 36 imperial gallons (160 L; 43 US gal). Wine was shipped in barrels of 119 litres (31 US gal; 26 imp gal). A barrel of oil, defined as 42 US gallons (35 imp gal; 160 L), is still used as a measure of volume for oil, although oil is no longer shipped in barrels. The barrel has also come into use as a generic term for a wooden cask of any size.