It's all about the mash bill. This isn't the name of a rock band (although it could be). The mash bill is the specific grain recipe for a whiskey. If you read the Den post about bourbon being an alcoholic example of American Exceptionalism, you learned its mash bill must contain at least 51% corn.
What happens once a bourbon distiller meets the 51% threshold? In theory, almost anything. I recently found this article on the Bourbon Of The Day website. The article provides an excellent overview of the four standard grains (corn, barley, rye, and wheat) and the three general recipes (traditional, high rye, and wheat) distillers use to create their bourbons. As the article astutely notes, there can be a lot of variations not just with the proportions of the grains, but also with the grains themselves.
So, with apologies to the Capital One ad with Samuel L. Jackson, what's in your bourbon?