Bourbon, which is also called mash bill, is composed of about 51% corn, with the rest of the mixture being rye, wheat, and/or malted barley. If the mash bill has wheat in it instead of rye, it is called a wheated bourbon. The grain is then ground and mixed with some water. Not always, but most of the time, some of the mash from a prior distillation is added to the mixture to make sure you have a consistent pH across the batches, and a mash that is made this way is called a sour mash. The final ingredient is yeast, and the mash is fermented. After it is fermented, this mash, known as the wash, is distilled to a concoction of between 65%-80% alcohol. Distillation was performed in the past using a pot still or alembic, but more modern usage calls for a continuous still to be used.
The resulting alcohol which is clear is placed into charred American oak barrels to age the spirit so that it gains both flavor and color from the caramelized sugars found in the charred oak. Changes during this time also happen because of chemical oxidation and evaporation. Bourbons tend to gain more flavor and color the longer the time of maturation. Maturity is the goal, not a particular age. Bourbon that is aged too long can become unbalanced and woody.
After the bourbon has matured, it is taken out of the barrel, usually diluted with some water, and then bottled to a minimum of 80 US proof. The most common bourbon whiskey is sold at 80 US proof. There are other common proofs of 86, 90, 100, and 107, but whiskeys of up to 125 proof can be sold. Some of the higher-proof whiskeys are sold as “barrel proof” meaning that they are undiluted or have been only slightly diluted after taking it out of the barrels. Bourbon whiskey can be sold at less than 80 proof but has to be labeled “diluted bourbon.”
The barrels afterwards still have 20 pounds of bourbon contained within the oak wood. They cannot be re-purposed for bourbon making, but are usually purchased by the Scotch whiskey industry.