M ark Twain once said, "There is no such thing as too much good whiskey." Chances are he was referring specifically to bourbon, a spirit he was known to adore. But what exactly is bourbon whiskey anyway, and what makes it so good?

Federal standards, issued by Congress in 1964, stipulate that bourbon must be a grain mixture made of at least 51 percent corn, produced in the United States, and distilled to no more than 160 proof, with nothing other than water added to the mixture (aside from yeast). It must also be aged in new, charred-oak barrels, among other requirements. The term "straight" bourbon designates whiskey that has been aged at least two years.

Distillers have had tremendous success experimenting with the mashbill (that's fancy distiller talk for "recipe"). Aficionados like to group bourbons into fancy-sounding categories. Below, a guide to the jargon and producers:

High Rye

Most bourbon mashbills contain corn, barley, and rye. The more traditional ones have 8 to 10 percent rye. A few bourbons stand out with a high rye content that makes for a bold, spicy flavor. Four Roses Single Barrel has the highest rye count of any bourbon on the market, at 35 percent. Other high-ryes include Bulleit, Jim Beam's Basil Hayden, and Old Grand Dad.

Flavors can vary: Bulleit is the fruitiest of them all. Basil (who was a pioneering distiller in the late 18th century) is the lightest of the Jim Beam Small Batch and has a nice spicy rye bite, but it lacks the full body of Bulleit. Old Grand Dad comes in three versions: 80 proof, 100, and 114 (that's Basil's image on the Old Grand Dad bottle). Both are made by Beam, so they are similar in flavor, but OGD's three variations, of course, get bolder the higher you go.

High Corn

Bourbon can be much more than the 51 percent corn minimum; many are made from 60 to 70 percent corn. But very few exist with a really high corn content. Buffalo Trace produces Old Charter with a corn content of more than 80 percent. The other main one is from New York's new microdistiller Tuthilltown Spirits, which made a splash in the market with its "Baby Bourbon"—made from 100 percent New York corn (yes, it's legal to produce the stuff outside of Kentucky) and aged in new charred wood. Whiskeys with a high percentage of corn may come across with extra sweetness. Also note that there is a separate and distinct category of corn whiskey; these spirits must be at least 80 percent corn and can only be aged in used charred barrels or new uncharred wood.

Wheaters

While most bourbons are made with corn, rye, and barley, a handful of outlaws defy that tradition with a mashbill of corn, wheat, and barley. Whiskey geeks call these "wheaters" or "wheated bourbons." This style offers a flavor profile with more pronounced caramel and vanilla and can be described as a tad softer. Maker's Mark is perhaps the most famous. Others include the many renderings of the Weller label, Rebel Yell, and any of the Van Winkle bourbons. Rebel Yell is the lightest and youngest of the bunch.

Small Batch

No legal definition exists to define "small-batch." Jim Beam's launch of Booker's, Baker's, Knob Creek, and Basil Hayden brought the term to the public mind. In general, "small-batch" means that a distillery used fewer barrels to make a bottling than it would have with its flagship brand, like Jim Beam white label. Still, this could mean that a distillery culled the choice stuff from 4 barrels or 4,000.

Maker's Mark draws on about 19 barrels for each batch. Some bottlings such as Noah's Mill or Kentucky Vintage label their bottlings with batch numbers and the dates that the bottling occurred. In this case, the batch number is an in-house number assigned to each dumped batch of whiskey to designate it from another set of barrels that may be dumped, mingled, and bottled at another time.

Single Barrel

Unlike "small-batch," the phrase "single-barrel" has real meaning. A single-barrel bottling is made from one barrel and bottled without mingling the bourbon with any other barrels. Single-barrel bottlings can be different from barrel to barrel, as each barrel offers different flavors depending on where it was stored in the warehouse and a variety of other characteristics, such as age and natural differences in the wood. Introduced to the world in 1984, Blanton's was the first single-barrel bourbon on the market. Many others exist now, such as Elijah Craig 18-year-old, Wild Turkey's Kentucky Spirit, and Eagle Rare. Elijah is the oakiest, smokiest of the three. Turkey's Kentucky Spirit has more chocolate and spice richness. Eagle is a good middle ground between the two, balancing 10 years in oak with a caramel smoothness. There is no relation to the phrase "single-malt" aside from the fact that both connote quality.