Bourbon refers to the product that the whiskey originated from. To be labeled as bourbon, a whiskey must be made from a mash that contains at least 51% corn. If you up that to 80%+ corn, it becomes corn whiskey.
Bourbon whiskey is corn whiskey mellowed by oak wood. It grew in popularity, in the 1800s and was produced in Bourbon county in central Kentucky. It came to be known as Bourbon whiskey. I wrote a hub on it about a year ago: http://hubpages.com/hubtool/edit/759521
Bourbon whiskey is a liquor made from a fermented mash of grain, containing at least 51% corn, distilled at not more than 160° proof. Straight bourbon, aged in charred oak barrels for two years in bonded warehouses, is a heavy, dry, mellow, full-bodied spirit. Blended bourbon, which contains neutral spirit, is lighter in body and in taste.
The first still for making corn-based whiskey was set up by Elijah Craig, a Baptist minister, in Georgetown, Kentucky, in the late 1780's. The liquor was named for Bourbon county, Kentucky. Bourbon, long a popular American drink, is a popular export.