There are two types of shochu:
Kourui 甲類 is made in a continuous still (renzoku shiki). It is very smooth on the palate and is under 36 degrees alcohol. Kourui shochu is typically used as a mixer for cocktails. On its own it does not have any notable aromas and is unimpressive on the palate. This is typically used as a cocktail mixer.
Otsurui 乙類, made in a pot still (tanshiki), are single distillation shochu. It retains the aroma of the base ingredient. The alcohol percentage is below 45 degrees. Otsurui shochu is good for drinking straight, on the rocks, or with hot water. It can also be used as a mixer for cocktails. Otsurui is also called honkaku shochu 本格焼酎. This is the top quality shochu that is worth exploring. If purchasing any shochu, be sure to ask if it is honkaku shochu.
Kojikin 麹菌 (aspergillus oryzae) is a mold that is used to break down the starches in the base ingredients into fermentable sugars. It is what makes shochu different from other distilled spirits. There are three basic types of koji that are used in creating shochu. The type of koji greatly affects the taste.
Shirokoji 白麹 (white koji) creates a very soft, gentle tasting shochu. These shochu are often light-body shochu.
Kikoji 黄麹 (yellow koji) is the same koji that is used for making sake. The resulting shochu is often aromatic with floral tones, and supple on the palate.
Kurokoji 黒麹 (black koji) is famous for making awamori (of Okinawa). Shochu made with kurokoji are often bold on the palate and full-bodied.
Another tip regarding shochu is to ask if it was distilled under high or low pressure. Genatsu 減圧 is distilled under low pressure, these shochu are often softer on the palate. Joatsu 常圧 is distilled under regular pressure creating more expressive shochu.