The stars don't collapse further because their electrons resist being pressed closer together. If a star has a mass greater than 1.4 solar masses, its gravitational forces are strong enough to overcome electron resistance, and it can form a supernova. A neutron star that had twice the mass of the sun would be contained in an area of about ten kilometers. The matter of a black hole is contained completely within a region called a singularity. Black holes all have certain other characteristics as well.
All black holes contain a horizon, which is the surface of the black hole.
Anything that enters this region cannot escape being sucked into the black hole. Black holes also rotate. This means that no distinguishing characteristics of a star can be determined from the black hole that it forms. For example, a spherical star and a non-spherical star will both form precisely spherical black holes.