Bourbon is an American whiskey, a type of distilled spirit,
made primarily from corn and named for Bourbon County, Kentucky,
which interestingly is a dry county. (You can make it, but not sell it there.)
It used to be a legal requirement that Bourbon came from Bourbon County.
This is no longer the case. Bourbon must meet several strict standards
in order to bear the name, though. They currently are:
Bourbon must be made of a grain mixture that is at least 51% corn.
Bourbon must be distilled to no more than 160 (U.S.) proof. (80% alcohol by volume).
Bourbon must be 100% natural (nothing other than water added to the mixture).
Bourbon must be aged in new, charred oak barrels. ("New" meaning they have never been used for any other spirit. Distillers of Scotch, Irish Whiskey and other spirits often buy used American Bourbon barrels to impart some of their residual flavor.)
Bourbon may not be introduced to the barrel at higher than 125 proof. (62.5% alcohol by volume).
Bourbon which meets the above requirements and has been aged for a minimum of two years, may be called "Straight Bourbon". This is an option, not a requirement, but most distillers of Straight Bourbon will proudly place it on the label.
Bourbon aged for a period less than four years must be labelled with the duration of its aging.
Almost all bourbons made today qualify as Straight Bourbons whether they are so labeled or not, with the exception of very bottom-tier brands or ready-mixed bourbon cocktail products. Even these must be aged a minimum of two years to carry the name "Bourbon" on the label.
Bourbons range from light and dry to dark and sweet, but common flavors present will include vanilla, maple, caramel, some smokiness, pepper or cinnamon notes and occasionally some fruity tones.
Like all whiskeys, Bourbon can be made from several distillation methods and blends of grain, though the bulk must still be corn. I encourage you to click on the distillation methods link for information on such terms as Single Barrel, Blended, Sour Mash, etc.