Tempeh is a fermented soy product. Although it may sometimes be associated with tofu, it is actually quite different, in that tofu is not fermented.
Tempeh begins with whole soybeans, which are softened and pulped; a culture is then added and the whole mixture is allowed to ferment for a day or more.
Tempeh has many nutritional and flavor components not achieved with the mere coagulation that takes place in tofu production. It is also high in fiber.
Other fermented soy products are miso and natto. It generally is made with soy, rice, salt or other soy-cereal-salt combination. In some recipes the fermentation take a long time, one year or more.