Miso (味噌) is a thick paste made by fermenting soybeans with salt. Often grains and sometimes other ingredients are added. It tastes salty and, depending on the type of grain used as well as fermentation time, more or less sweet. Miso is often eaten as miso soup.
The grains used include any mix of barley, rice, rye, wheat among others. Fermentation time ranges from as little as 5 days to many years. The wide variety of Japanese miso is difficult to classify but is commonly done by grain type, color, taste and region.
Much like yoghurt, miso contains live bacteria of a highly benefical nature and can be used to repopulate a person's intestine with beneficial bacteria after taking antibiotics.
Some, therefore, say that miso should never be cooked, but always added to food after it has cooled to eating temperature. This view is not common in Japan, where a variety of cooked miso dishes (miso soup, dengaku (charcoal-grilled miso covered tofu), miso braised vegetables or fish, and so on) have been made for many hundreds of years.