A white dwarf, also called "degenerate" star, is a "dead" star which has spent its fuel of hydrogen and then helium, converting the bulk of these into elements like carbon, oxygen and nitrogen. It is ultra-dense, having collapsed much of the space within the matter composing the burnt out husk.
This is the end stage of stars in the medium-small mass range roughly equivalent to that of our sun. Too small, and the star (a red dwarf) will not have enough mass to compress into a white dwarf.
A more massive star will end its life as either a neutron star or black hole, likely after a supernova explosion.
Some white dwarfs result in supernovae because of massive amounts of matter being dumped on the super hot, dense dwarf.
After billions of years, perhaps even trillions, the white dwarf will cool off becoming entirely cold and dark -- a black dwarf, not to be confused with "black holes."