Bourbon is an American corn based whiskey.
The Code of Federal Regulations defines bourbon as follows:
(1)(i) "Bourbon whisky", "rye whisky", "wheat whisky", "malt whisky", or "rye malt whisky" is whisky produced at not exceeding 160° proof from a fermented mash of not less than 51 percent corn, rye, wheat, malted barley, or malted rye grain, respectively, and stored at not more than 125° proof in charred new oak containers; and also includes mixtures of such whiskies of the same type.
Although bourbon gets its name from Bourbon County in Kentucky, the primary producer in the 1700s and 1800s, it can now be produced legally all over the United States. Most of it still come from Kentucky, though.