A black dwarf is a star in its final stages after it has used its energy supplies as a white dwarf. Black dwarfs are no longer luminous and I hear they're "cold". Theoretically, there aren't any as the universe isn't old enough for any to have developed.
I'll try to remember to update this writeup once it is old enough. :-)
Update: The Universe just keeps getting older. I don't know how old it'd have to be to allow for black dwarfs but it's now (April 2000) estimated to be about 14 billion Earth years old. | 
