Years ago before there were any laws on making whiskey, backwoods distilleries were making whiskey with whatever means they had. Some places would use old nail or fish barrels to store the whiskey in to ship and then people drinking it were getting sick. The government stepped in and made some laws governing the production of straight whiskey or bourbon.
To stimulate some of the different economies hurting at the time, the United States government said that bourbon or straight whiskey must be made with at least 51% corn, which helped out the farmers. Rye, wheat, corn and/or barley are usually what makes up the other 49% (each distillery has their own sometimes secret recipes). It was also required to be aged for no less than two years in brand new, charred, white American oak barrels, which stimulated the logging industry. Also bourbon has to be at least 70 proof.
Most bourbon is still made in Kentucky but there are more whiskey distilleries being established around the country. We have straight bourbon whiskey from at least 17 states in stock currently with the number increasing all the time due to the boom in micro distilleries.