The universe contains three different categories of black holes: stellar, supermassive and miniature, which are further divided according to whether they are spinning. Non-spinning black holes are always spherical, while spinning black holes tend to be more oblate. The degree of equatorial bulge is determined only by the rotational velocity of the object.
Stellar black holes are the result of a star in excess of around three times the mass of the sun collapsing beyond the point of no return under its own gravity. The collapsing star rapidly becomes so dense that its escape velocity rises above the speed of light, and it forms an event horizon.
Supermassive black holes have a mass of billions of suns and are found in the centers of galaxies. It is not known exactly how these black holes form, but they are known to grow by consuming thousands of stars surrounding them. The frictional heating of these torn-apart stars spiralling into the black hole often casts off twin jets of radiation and charged particles that can be millions of light years long.
Miniature, or primordial, black holes are a leftover from the formation of the universe. They are theoretical, in that none have actually been discovered. In the early universe, it is probable that forces crushed relatively small amounts of matter together to critical density. These small black holes would have about the mass of a large mountain and the diameter of an atom.