Whiskey is an alcoholic beverage made from grains (typically barley, but also other grains). Whiskey must have an alcohol content of at least 40% and must be matured for at least three years in the EU.
Whiskey is available as single cask, single malt, blended, grain. Single cask is whiskey from a single cask. Single malt is from one distillery. Blend is from at least two distilleries and may contain grain. Grain is made from something other than barley - typically corn or wheat.
A blended whiskey usually contains whiskey from several different distilleries, but can also be a mixture of grain and malt whiskey from a single distillery. It is produced in a number of countries around the world, but Scotland and Ireland are most often associated with the production of whisky.