First off, whiskey and whisky are the same thing with different spellings. In Scotland it’s spelled “whisky” while in Ireland it’s “whiskey” with an extra “e”. The US chose the Irish spelling, so “whiskey” also refers to American varieties.
Regardless of spelling, whiskey is a distilled alcoholic beverage made from fermented grain mash (the grain can be barley, corn, rye or wheat) and typically aged in charred white oak barrels. Mash, by the way, is the mixture of the grain, water and yeast that is fermented to produce the alcohol content. Varying availability of grains in Scotland, Ireland and the Americas historically led to experimentation with different kinds of mash, resulting in the differences we have today. As a result, today’s whiskeys are categorized and marketed depending on how and where they are made.
Scotch is a whisky (without the “e”) that can only be made in Scotland from mostly malted barley, and aged in oak for at least three years. There are a wide variety of Scotch whiskies known for unique and identifiable flavors, most notably single malt whiskies from isle of Islay.
Bourbon is a whiskey that is distilled from a grain mash that is at least 51% corn, must be aged in charred oak containers, and cannot contain any additives. It’s generally associated with Kentucky and Bourbon County, although “Tennessee Whiskey” also meets the requirements to be called bourbon.
Rye is a type of wheat closely related to barley. Rye whiskey made in the US must be distilled from grain mash that is at least 51% rye and aged at least two years.
You might have heard of sour mash whiskey. This refers to a common production process that American distilleries use to regulate unwanted bacterial growth. It doesn’t make the whiskey sour, and it doesn’t always get marketed on the label. The process adds “spent mash” leftover from a previous distillation run to a new batch of mash. Spent mash is acidic and nutrient-rich and helps control the pH of the current mash to avoid unwanted bacteria.