The difference in "Tennessee Whiskey" is the charcoal filtration. They pour the whiskey over an enormous pile of charcoal, which provides a unique flavor, independent of standard bourbon. They obtained the legal right to distinguish it this way a long time ago. To me, technically, Tennessee Whiskey could be called bourbon, but bourbon should not be called Tennessee Whiskey without this process. IE, square/rectangle relationship.
After the charcoals, it goes to the barrels, and anything that ages appropriately (usually depends on the height of the barrel in the storeroom and the strength of the winters/summers) between, I think, 3 and 7 years becomes JD black label. If it hasn't aged after 7 years, it becomes green label. Gentlemen Jack gets put through the coals twice, and single-barrel is well, single-barrel.
If you haven't been to Lynchburg, I recommend it. It's an awesome tour.