Whiskey is a drink made by distilling grains and aging the resulting alcohol in oak casks. It may also be spelled whisky, particularly when describing Scotch, Welsh, or Canadian whisky. Irish and American whiskey are usually spelled with the added "e." The name is derived from an Irish Gaelic term, uisge beatha, which is often translated as "the water of life."
Like many grain alcohols, whiskey may be made using an assortment of different grains, including rye, barley, corn, and wheat. It is sometimes referred to as the "juice of the barley," an allusion to the grain used in creating that type of whiskey. Malt whiskey is made entirely from malted barley as the source grain, and a single-malt whiskey is one in which all the malt comes from one stilling, rather than blending many different batches together.
Bourbon is a type of whiskey in which more than half of the grain used is corn, and it is often treated as its own distinct alcohol. It is aged for at least two years in a white oak barrel to let it mature before being bottled. Usually, bourbon is adjusted from its high alcohol content of around 160 proof down to between 80 and 100 proof before being sold, though some bourbon is sold at cask strength, in which case it remains at the proof it leaves the cask after aging, often as high as 120 proof.
Irish whiskey is generally made from barley, either entirely from malted barley, in which case it is a single-malt, or from a blend of unmalted and malted barley, in which case it is known as pure pot still. Blended whiskeys from Ireland may also include corn or wheat alcohols mixed in with a blend of barley.
Scotch whisky is often simply referred to as Scotch in the United States and elsewhere, or simply as whisky in Scotland. A single-malt Scotch is made entirely from malted barley from one distillery; a blended malt uses all malted barley, but may mix batches from different distilleries. A single grain Scotch uses alcohol all from one distillery, though it may blend different grains, while a blended grain may use multiple grains from multiple distilleries. Of these, the single-malt Scotch is usually hailed as being of the highest caliber, and as such is both the most expensive and the most sought after.