Barrel also called as cask, is a hollow cylindrical container, traditionally made of vertical wooden staves and bound by wooden or metal hoops. Barrels often have a convex shape, bulging at the middle and this makes it easier to roll a well built wooden barrel on its side, changing directions with little friction. Barrels used for maturing are generally made up of oak and some times with cherry, walnut, chestnut, pine and redwood etc.
Wine:
Wine can be fermented in wooden, steel etc and an aging barrel is used to age the wine. Wine is specially fermented in the barrels which are made up of oak as it is composed of chemical compounds like tannin that gives wine its vanilla, tea and tobacco like characteristics. Wine can be fermented in stainless steel tanks and rich vanilla flavor is added to the wine if they are fermented in the stainless steel and then it is aged in oak.
Sherry:
It is stored in 600 liter barrels that are made of North American oak, which is slightly more porous than French or Spanish oak. The casks, or butts, are filled five sixths full and a space of two fists is allowed at the top to allow flor to develop on top of the wine.
Whiskey:
Whiskey is stored according to the laws in the respective countries. For example, law in the United States requires whiskey must be stored for at least 2 years in oak containers. International laws require any whiskey bearing the label Scotch to be distilled in Scotland and matured for a minimum of three years and one day in oak casks.
Cognac:
To get a good cognac it is very important to mature it in oak casks. The used wood for those barrels is selected because of its ability to transfer certain aromas to the spirit. This ages only in the barrels made up of oak which are derived from wood from the Tronçais and more often from the Limousin forests.
All the above beverages use barrels for maturing.