According the Hertzsprung-Russell diagram, a white dwarf is a non-main sequence star, supported by electron degeneracy. Some white dwarfs are blue, rather than white. Many white dwarfs are approximately the size of Earth.
A star like our Sun will become a white dwarf when it has exhausted its nuclear fuel. Near the end of its nuclear burning stage, such a star goes through a red giant phase and then expels most of its outer material (creating a planetary nebula) until only the hot (T > 100,000 K) core remains, which then settles down to become a young white dwarf.
A typical white dwarf is half as massive as the Sun, yet only slightly bigger than Earth. This makes white dwarfs one of the densest forms of matter, surpassed only by neutron stars. The higher the mass of the white dwarf, the smaller the size. There is an upper limit to the mass of a white dwarf, the Chandrasekhar limit (about 1.4 times the mass of the Sun), after which the pressure of the electrons is no longer able to balance gravity, and the star continues to contract, eventually forming a neutron star.
Despite this limit, most stars end their life as white dwarf, since they tend to eject most of their mass into space before the final collapse (often with spectacular results, see planetary nebula). It is thought that even stars 8 times as massive as the Sun will in the end die as white dwarfs.
White dwarf stars are extremely hot, hence the bright white light they emit. This heat is a remnant of that generated from the star's collapse, and is not being replenished (unless they accrete matter from other closeby stars), but since white dwarfs have an extremely small surface area from which to radiate this heat energy they remain hot for a long period of time.
Eventually a white dwarf will cool into a black dwarf. Black dwarfs are ambient temperature entities and radiate weakly in the radio spectrum, according to theory. However, the universe has not existed long enough for any white dwarfs to have had enough time to cool down this far yet and so no black dwarfs are thought to exist.
Many nearby, young white dwarfs have been detected as sources of soft X-rays (i.e. lower-energy X-rays); soft X-ray and extreme ultraviolet observations enable astronomers to study the composition and structure of the thin atmospheres of these stars.
White dwarfs cannot be over 1.4 solar masses, the Chandrasekhar limit, but there is a working method to get them over this limit. Touching on a nova, a white dwarf can accrete material from a companion. Unlike a nova, the material accretes slowly and remains stable. The mass of the white dwarf increases until it hits the 1.4 solar mass limit, at which degeneracy pressure cannot support the star. This is a type I supernova and is the most powerful of all the supernovae.
White Dwarf is also the name of a magazine.