Unlike most other beans, fermented black beans (aka salted black beans, douchi, touchi) are used more as an umami-rich seasoning than as a base ingredient.  Made from cooked and fermented black soybeans that have been salted and sun dried, fermented black beans have a powerful salty/bitter/sweet flavor and spicy/musky aroma.  They are used to flavor sauces and other dishes in many Asian cuisines.

Fermented black beans are best stored in a cool, dry cupboard or the refrigerator, and will last a very long time.  Because of their strong odor, care should be taken not to store them near any food that absorbs odors easily (like butter).

Fermented black beans are usually soaked in water for about 30 minutes to rid them of excess salt, and lightly pressed (perhaps with the side of a knife) prior to use.  This releases their full flavor.  They can also be blended with other ingredients to make black bean paste.

In Chinese food, they’re often used in stir fry dishes, particularly as part of seafood sauces (particularly for squid, mussels, or clams), meat sauces, and sauces for green vegetables including broccoli and bitter melon.  They work very well with garlic.
 
Outside of traditional Chinese fare, black beans can also be finely minced with garlic and ginger to make a flavorful meat or fish seasoning/rub.