If you go to a good sushi restaurant anywhere in the world, you will often finish your meal with a small bowl of miso soup. Miso soup is so important in Japanese cuisine that people used to say that you can see that someone would be a good home cook if their miso soup tastes good. In the past, every Japanese household would always have a pan of miso soup brewing on the stove.
The original form of miso is said to have already existed in the Nara period (710-794). In the Heian period that followed stores selling miso to the general population started popping up. Miso is made from soybeans, which are steamed and mixed with salt and koji-fermented rice, barley, or beans. It used to be made at the temple in the Kamakura period (1185-1333), and at the end of civil war in the 15th and 16th centuries, it was used in rations for soldiers. It was in the Muromachi period after that when miso became widely popular just like soy sauce did around the same time.
Miso is classified as salty miso and sweet miso according to the percentage of saltiness. While the salt contents determines the saltiness, the sweetness is determined by the proportion of malt. Generally, miso from the northern part of Japan is salty and the sweet version comes from the southern part of Japan.
There are also two miso colors, aka miso (red) and shiro miso (white). The color is determined by the type of soybeans that are at the base of the product, the production process, and the length of the fermentation period. In Tokyo, people prefer white miso and this is also the most widely used type, but in Nagoya people strongly prefer red miso. The flavor of red miso tends to be a lot stronger, so if you are eating dishes that are heavy on miso, white miso tends to be good for starters, and if you are feeling bold, trying the red miso can be a good experience.
Because miso is a fermented food, it is healthy like cheese and other fermented foods. Miso is not only used for soup, but also for dishes like miso-braised vegetables, grilled miso, many sauces and dressings, and as a side dish. Miso can either be bought at regular supermarkets and convenience stores and at specialty stores that tend to have more options.