Whiskies are produced across the United States, though the most well-known states today for American Whiskey production are Kentucky and Tennessee (many would be surprised to learn that the first producers were based in Pennsylvania and Maryland!). American Whiskey is a vast category, including Bourbon, Rye, Corn, Wheat and Tennessee among others. Most American Whiskies are aged in charred new barrels, offering a characteristic toasted component.
The main differentiators have to do with the base cereals. Bourbon must come from 51-80% corn, with the remainder made up of wheat, rye and/or barley) distilled to a maximum of 80% ABV and aged in new charred white oak barrels. The high component of corn is a contributor to the spirit's characteristic sweetness. Rye must be at least 51% rye, resulting in general in a more spicy whiskey than Bourbon. Wheat must be at least 51% Wheat and is often a softer style of American Whiskey. Tennessee Whiskey is at least 51% corn, like Bourbon, but must be produced in Tennessee and goes through maple charcoal filtering ("the Lincoln County Process") before bottling. Perhaps the most unique and trending American Whiskey style is White Dog, also known as "Moonshine", which is an unaged whiskey bottled right after distillation.