What is the difference between black and white dwarfs?
Below if the difference.
A White Dwarf is the last stage of stellar evolution. A Star much like our Sun, a main sequence medium sized star after burning all of its hydrogen into helium will calmly shed its outer layers into a planetary nebula. After this it will transform into a White Dwarf a Small extremely dense Star about the size of the Earth.
This White dwarf will continue to radiate heat and energy for the next 10 to 100 Billion years until it can no longer give off radiation. This is hypothesized as a Black Dwarf. Black Dwarfs are only hypothetical because, even if this stage occurs, the universe is still young to have any black dwarfs. Also they will be extremely difficult to find as they won't give off heat and energy and also they will be Black.