Jack Daniels is a Tennessee whiskey, not a bourbon. The difference between the two comes during the final aging process in oak barrels. Jack Daniels is available in two separate labels, the Black and the Green. The Green Label is aged for a shorter time than the Black.
Process
To complete the aging process, the whiskey is dripped through 10 feet of a special type of charcoal made from hard sugar maple, into a charred oak barrel. The company claims not to have a set time for the aging process. Instead, it employs professional tasters to determine when the whiskey is ready.
Green Label
Jack Daniels Green Label has a lighter flavor and color than it's counterpart. The character of the whiskey is smoother, and the overall taste is mellower than the Black. The Green Label is not aged as long as the Black. It is stored on the lower floors in the Jack Daniels warehouse, a position that retards the aging process, producing a less mature version of the whiskey.
Black Label
The Black Label, like the Green, goes through the process of charcoal filtering, and is stored in charred oak barrels. However, these barrels are not stored in the center of the warehouse which expedites the maturing process. The flavor is stronger than the Green Label and the color is darker, making the character more complex. Black Label is more expensive than the Green Label.