For a whiskey to qualify as bourbon, the law–by international agreement–stipulates that it must be made in the USA. Bourbon whiskey follows the same geographical significance that scotch does, it is only bourbon if it is made in Bourbon County, Kentucky as scotch is only scotch if it is made in Scotland. It must be made from at least 51% and no more than 79% Indian corn, and then aged for at least two years. (Most bourbon is aged for four years or more.) The barrels for aging can be made of any kind of new oak (charred on the inside). It must be distilled at no more than 160 proof (80% alcohol by volume). Nothing can be added at bottling to enhance flavor or sweetness or alter color. The other grains used to make bourbon, though not stipulated by law, are malted barley and either rye or wheat.
The same style of whiskey can be made anywhere, but it can’t be called bourbon. Those are simply industry standard, most of distilleries age bourbon for a much longer period of time and use up to 75 percent corn grain.
Types of bourbon tend to vary from small batch to single barrel and straight to blended. Small batch bourbons are bottled from a premixed batch of barrels, whereas single barrel bourbons are all bottled from a single barrel – pretty self-explanatory. Straight bourbons are produced from the sour mash method with liquid from an earlier distillation, which is then passed onto the next distillation in order to produce a consistency between batches. Blended bourbon, on the other hand is a mixture of 51 percent straight bourbon with other "grain-based spirits".