Three types of black holes have been discovered over the past 25 years, first by theoretical physicists and then by observational astronomers.
The first kind of black hole is caused when a star of about 20 times the mass of our Sun goes supernova and then its remnants, mostly neutrons, are so dense that they collapse in on themselves, because the gravitational forces are no longer balanced by internal release of thermonuclear generated energy (the star has "winked" out). The resultant gravitational collapse forms a hole in space-time that will "eat" all matter that crosses its surface (the event horizon), including photons, so no light can escape, and drag it into an infinitely dense point in the hole called a singularity. What happens thereafter to the ingested matter is anyone's guess, since all the laws of physics, relativity and quantum mechanics have broken down and no longer work. A new theory of guantum gravity is being developed to explain the laws of nature that govern singularities
Some theoretical physicists, most notably, Steven Hawking, believe that the singularity ejects the matter into another universe as a "Big Bang" or into another dimension, but there is, of course, no proof of this and almost any theory of what happens in the singularity is viable at this time.
A second type of black holes is called a Supermassive black hole which forms at the center of most galaxies, including our own, not from a collapsed star, but from collapsed interstellar gas and other matter that has become so massive as to allow its gravity to force it into a black hole. These galactic monsters are thousands of times bigger than the ordinary black hole and they seem to be an integral part of the life cycle of galaxies, rather than the death throes of an individual star.
The third type of black hole is found in stellar nurseries billions of light years away from us (and hence existing in the early years of our universe) and are identified by the violent bursts of gamma radiation they release, which can be seen on earth when the hole's event horizon is facing us. The function of these types of holes is unknown but they form from Hyper-novas, the explosion of supermassive stars that are too big to survive more than a few million years (as opposed to to the ten billion year life span of most other stars). These Gamma Ray Bursters may be involved in the birth of second generation stars in those stellar nurseries.
It has also been postulated that there exists throughout the universe microscopic black holes, but this has not yet been observed in even our most powerful extant "atom smashers", and their verification will await experiments in the next generation of supercolliders now being planned.