What exactly is a white dwarf

I know that when a star dies it can become one of quite a few things, black hole, white dwarf etc...

I read somewhere that a white dwarf is a giant diamond, the biggest known to man but that was a bit vague for me. So what is a white dwarf, I would like detailed answers please, only answer if you know what your talking about.

A white dwarf is not really a star at all, as it no longer creates heat and energy through fusion. It is the burnt out core of a dead star, kind of like a glowing ember after a fire, its not active, but it still gives off heat and energy. Only stars about the size of our own sun become a white dwarf (after the red giant stage, it sheds it's outer layers to form a planetary nebula with it at the center)
It is super dense and because the star (through fusion) created carbon at it's core, the pressure collapsed the atoms into a crystal structure.

It's just a small star that burns white. That's the basic idea anyways.