Since it's easy to come by, it wasn't long before the early Kentucky settlers had far more corn than they could consume. Certainly a lot went to feeding livestock, but some resourceful soul soon came up with the idea of using it to make an alcoholic beverage.
It's the method of aging Bourbon which largely defines its taste. Some genius charred the inside of the barrel, probably by building a fire inside to assist in the bending of the wet staves during its construction and that charring flavored the whiskey. Bourbon's sweetness comes from the corn, its complexity of flavors from the other grains such as barley and rye, but the vanilla and caramel comes from the barrel.
Long aging will smooth the whiskey, but since the law requires Bourbon to be aged in new oak barrels, an inordinate amount of time in the barrel can result in a lot of wood flavor being imparted. That's why you'll probably never see a 25-year old Bourbon. 15-16 years of aging is the maximum for a smooth, balanced result.
Another characteristic of Bourbon is the use of sour mash. Most whiskey makers add back a bit of the residue from the distillation to the new batch which helps guard against unwanted yeast taking over the fermentation and, to standardize the taste from one batch to another. Some Bourbons place the words 'Sour Mash' on the label and some don't. But virtually all use it, one way or another.
The preferred style of distillation for Bourbon is a column still which runs a continuous stream of distillate through it, rather than a pot still which rectifies one batch at a time; the method used for single-malt Scotch and Cognac, for example.
The term 'straight' as applied to Bourbon, such as "Kentucky Straight Bourbon Whiskey" has a significant meaning. What it's telling you is that the Bourbon hasn't been 'stretched' with the addition of neutral grain spirits, thus all the whiskey in the bottle was made in the traditional way.
A whiskey which has been 'bottled in bond' must be at least 100 proof in alcoholic strength. This was the designated level when the law was created to permit distillers to store packaged spirits and not pay the tax on it until sold. Today, most whiskeys bottled at 100 proof are called 'bonded' whiskies.