Well, a white dwarf is a star that has due to old age either shed it's outer layers leaving only it's core, or s star that had enough mass to collapse after going supernova but not enough to colapse into a black hole or Neutron star. A nebula is a large clustering of mainly hydrogen gas and a few other atoms, useually the remnants of a super nova or still unformed remnants of the big bang, that forms new stars. (yes, my answers are long and complicated because I know the subject, your don't have to be, you could have said something as simple as one is a star and the other is a gas cloud)
So let's get out of space and into a more earthly matter: Granite and Limestone.