Single Barrel Bourbon
The idea is that each barrel of Bourbon is a little different, and the whiskey in its acquires a slightly different flavor. Furthermore, where store the barrel is stored in the warehouse can make a considerable difference in how the whiskey ages. Typically, when a distiller bottles a Bourbon, he collects together several hundred or even thousand barrels. These are dumped together, chilled and filtered, cut with water, and then bottled. What results is a reliably uniform whiskey. What the master distiller does, then, is to periodically sample the whiskey in barrels, especially those barrels in the best part of the warehouse. Usually this is the very center of the warehouse, called 'the heart' of the warehouse. Those barrels that are discovered to contain unusually fine whiskey are recorded and tracked with care. They are allowed to mature in years far beyond the average Bourbon. When they are at their peak of perfection, they are taken from the warehouse -- one by one -- and bottled one barrel at a time. In this way the cream of the crop, so to speak, becomes single barrel Bourbon.
Small Batch Bourbon
Small batch bourbon is made slowly with carefully chosen oak barrels. Rather than bottling Bourbon from a single barrel or from a large batch comprised of many barrels, a few barrels are combined into a batch that is bottled and labeled with an individual batch number. Small batch Bourbon is generally not diluted with water to make a standard "proof" and thus has higher alcohol content, generally from 120 to 160 percent (60 to 80 proof), than large batch Bourbon. The barrels for the small batch are individually chosen by the distiller for their quality and some small batch makers do not filter their Bourbon, making a darker, richer finished product.