Shoyu is actually the Japanese word for “soy sauce.” In Japan, there are a number of different forms of shoyu, which are differentiated on the basis of ingredients and fermentation technique. In the West, there is some confusion about what shoyu is, compounded by the use of “Shoyu” in the branding of several soy sauce companies, including the Aloha Shoya Company in Hawaii.
All soy sauce is made from fermented soybeans. Depending on how they are handled and how long they are fermented, the soy sauce may be light in color and texture or inky black. Different soy sauces are designed to pair with different foods; in Japan, some famous types are tamari shoyu, made by pressing the soybeans used to produce miso, and saishikomi, a double brewed shoyu which is extremely strong.
The Japanese inherited the soy sauce manufacturing tradition from China, a nation which has been making soy sauce for thousands of years. Japanese techniques for soy sauce manufacture date to around the 1600s, and several other Asian nations have developed their own soy sauces and other fermented sauces like fish sauce. The fermentation of soybeans creates a rich, complex flavor and helps to preserve the sauce, ensuring that it will not go bad in unrefrigerated conditions.
When soybeans are fermented to make shoyu, they are packed with sea salt, which acts as a preservative, and koji, a type of mold. Wheat or another grain is often added to temper the flavor; most Japanese shoyu includes wheat, which lends a sweet and slightly alcoholic note to the finished soy sauce. When Japanese-style soy sauce was first introduced to the West, it was sometimes mistakenly labeled as "tamari," which led to general confusion when actual tamari was introduced.
Many consumers in the West think of soy sauce as a watery, extremely salty brownish liquid which accompanies sushi and Chinese food. In fact, soy sauce is incredibly complex, and much like wines, soy sauces come in a range of flavors and qualities. Really good soy sauce is comparable to fine wine for connoisseurs of Asian foods, and it can fetch a very high price. Just like wine, the flavor of shoyu varies depending on the vintage, how it is handled, and how long it is allowed to age.