A barrel or cask is a hollow cylindrical container, traditionally made of vertical wooden staves and bound by wooden or metal hoops. Traditionally, the barrel was a standard size of measure referring to a set capacity or weight of a given commodity. For example, a beer barrel had originally a capacity of 36 US gallons (140 L) while an ale barrel a capacity of 32 US gallons (120 L). Wine was shipped in barrels of 31.5 US gallons (119 L). A small barrel is called a keg.

Modern wooden barrels for wine-making are either made of French common (Quercus robur) and white oak (Quercus petraea) or from American white oak (Quercus alba) and have typically these standard sizes: "Bordeaux type" 59.4 US gallons (225 L) and "Cognac type" 79.2 US gallons (300 L). 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. Barrels are only one type of cooperage. Other types include, but are not limited to: buckets, tubs, butter churns, hogsheads, firkins, kegs, kilderkins, tierces, rundlets, puncheons, pipes, tuns, butts, pins, and breakers.