A white dwarf is a stage in the life cycle of a star, before it cools off to become a black dwarf, emitting no energy. The Earth's sun is a star, and it too will one day become a red giant and eventually a white dwarf. A white dwarf is not as huge as the sun, but is only slightly larger than the planet Earth.
The core of a white dwarf can range around 100,000K or more and is very densely packed together. According to Windows to the Universe online, a spoonful of a white dwarf would weigh as much as a car on Earth.
White dwarfs are so dense and burn at such a hot rate, that it takes hundreds of billions of years for it to cool down completely. It takes roughly 200 million years just for a white dwarf to lower its temperature from 100,000 degrees K to 20,000 degrees K.