Bourbon whiskey must be made from a grain mixture which is at least 51% corn. The fermentation process for this mixture is often started by mixing in some mash from an older already fermenting batch, a process known as sour mash. Much like how Scotch must be made in Scotland, Bourbon can only be labeled as Bourbon if it was made in the United States. While the rules are slightly more loose with Bourbon than with Scotch it still has to conform to a few requirements. The spirit must be distilled to no more than 80% alcohol (160 proof) and be no more than 62.5% when put into casks for aging in new charred oak barrels. Finally Bourbon has no minimum aging period, but to call your product Straight Bourbon it must be aged for no less than two years (and can have no added coloring, flavor or other spirits added).

Basically, scotch whiskey is distilled in Scotland from grain or malt. Bourbon is distilled mostly from corn in the United States.