there is an upper limit to the mass of stars above which two things are possible-
1.explosion of the star to form neutron stars called PULSAR.
2.collapse and compaction of the star to form BLACK HOLE
therefore black holes are formed due to collapse and compaction under gravity at the end of the life cycle.
it is said that the gravity pull in black hole is this much that even light cannot get out of it.
the supermassive black holes are largest black holes that have masses more than 1 million suns together.
one thing is that our sun cannot ever become a black hole as it is not big enough.NASA is also sending satellites into space to learn more about black holes.