Straight bourbon whiskey is all about consumer protection, aging, and proof. The Pure Food and Drug Act of 1906 dictated that "straight whiskey" must be distilled from a fermented cereal grain mash, may not be distilled higher than 160 proof (80% ABV), aged for a minimum for 2 years, and enter the barrel no more than 125 proof (62.5 ABV). Only water may be added to bring the proof down.

Last, any straight whiskey aged less than four years must have an age statement, and it always reflect the youngest whiskey in the bottle.

You'll notice the original question was about "straight bourbon whiskey" and the Pure Food and Drug Act of 1906 dealt mainly with "straight whiskey" that's because bourbon wasn't officially bourbon until 1964. That's why you'll also notice some of the "straight" requirements mirror bourbon's like the distillation and barrel entry proof.

To be designated, "straight bourbon whiskey" a spirit must not only meet all of the requirements from the Pure Food and Drug Act of 1906 but also meet Congress' 1964 definition of what bourbon is.

Also, in common parlance, "straight" can also be a request for a whiskey or bourbon without ice or other add ons at a bar. "Hey bar keep, give me a shot of Maker's straight."

The types might fall into different categories. We will start with the different types of bourbon mashbills.

The most common bourbon mashbill has the required 51% corn, a secondary grain of rye, and barley.

This mashbill can be high rye (e.g., Wild Turkey, New Riff) or low ry (e.g., Elijah Craig, Buffalo Trace). The higher the rye generally the more spice is present.

The next most popular mashbill replaces the rye with wheat. So you might 51% plus corn, wheat, and barley. 

This mashbill was made popular by the now defunct Stitzel Weller distillery in the mid-twentieth century with brands like Old Fitzgerald, Rebel Yell, Weller, and Pappy Van Winkle.

These wheated bourbons are still being made today but by different distilleries after Stitzel Weller shut its doors in 1972. The wheat is a softer flavor and takes premium aging well.

Let's call the last type experimental. Some distillers are more adventurous with their mashbills than others. You will have mashbills that include four grains (corn, wheat, rye, and barley), five grains, or other grains like amaranth, malted grains, triticale, rice, oat, millet, or sorghum.

The other types of bourbons would fall into a category we've touched on already — straight bourbon whiskey and bottled in bond. 

These terms can be confusing for bourbon newbies. Single barrel is the easiest to define so we'll start there. 

After the spirit is distilled, it's moved to barrels where it becomes bourbon whiskey. Some barrels may have exceptional flavor on its own. 

Distilleries market and sell this bourbon whiskey as a single barrel. This designation was popularized by Colonel Albert Blanton at the now Buffalo Trace Distillery and is sold as Blanton's. 

It's a single barrel of bourbon whiskey not mingled or blended with any other barrel.

Small batch is less clear and can cause confusion. There are no regulations on what small batch means. It might mean a craft distillery mingling 5-10 barrels to hit their desired flavor profile. Or it might mean a larger distillery mingling 200 plus barrels.

Some popular small batch bourbon whiskeys include Maker's Mark, Buffalo Trace, and Elijah Craig.