A black hole forms when a very massive star explodes, leaving behind a very dense remnant core. If the star is massive enough, the core will be compressed to a very small volume with extreme density, generating a powerful gravitational force that is characteristic of black holes.
a black hole is formed when a star much larger in mass than our sun begins to create iron in its core, and goes supernova. the star itself implodes and sends the energy outward. but if the core collapses completely, then time "halts" at the moment of complete collapse at the event horizon, but beyond that, the star continues to collapse until it creates a singularity.
thus, a blackhole is born.
A very huge star (50 times as big as ours, billions of times the size of earth) "burns" enough of it's hydrogen fuel off by converting it into nuclear energy, radiation (heat and light) that it's mass is diminished, which means it's gravity is diminished, enough so that it is not enough to hold back the extreme explosive force of the nuclear fusion going off in it's core (imagine millions of nuclear bombs going off at once, that is what is at the core of every sun... literally). So it's explosive power finally outwrestles it's gravity and it explodes, but because it's mass is still so massive at this point it collapses back in on itself, so dense that it's gravitational force multiplies, collapsing matter and pulling in everything that comes too close.
Black holes, once considered hypothetical bodies, are detectable through their effects on celestial bodies as well as the intense radiation given off by matter passing by not close enough to be pulled in, but not far enough to escape, which "slingshots" around and around the black hole's gravitational field emitting large amounts of invisible X-ray radiation, visible by special x-ray sensitive telescopes billions of light-years away orbiting earth.