Fermented black beans are small, black soybeans that have been preserved in salt. Also known as Chinese black beans or salty or salted black beans, they have a very strong, salty flavor and are generally soaked for a half hour or so in fresh water before being added to a dish.
Fermented black beans are not the same thing as black bean sauce, although you can substitute the sauce for the beans in a pinch. The sauce is much more widely available in supermarkets, but you should look in the ethnic foods aisle anyway, and any Asian market will carry them.
Finally, fermented black beans are not to be confused with plain ol' black beans, which are different beans, processed a different way.