Many do not know the true nature of a black hole; most believe that it is just a massive body that sucks things in. In essence, that is true, but there is more to it. A review of black hole formation comes from the article on supernovas. A black hole is the result of the collapse of a very massive star. Under normal conditions, a star will burn hydrogen fuel, converting it into helium. During the last stages of the star’s life, when the hydrogen fuel runs out, the star begins to burn helium into a heavier element. These elements that are burned, other than hydrogen, create an imbalance between the gravitational forces and the nuclear forces that under normal conditions keep the star stable (see supernova). Due to this imbalance, gravity takes over and the star begins to collapse upon itself until it reaches a point of infinite density and infinitely small size, in other words, a singularity. This singularity, now known as a black hole, creates a massive gravitational effect unlike any other. This effect is so great, that even light cannot escape its gravitational wrath.
A black hole also has an event horizon, the boundary of the black hole where anything inside will feel the effects of gravity. Anything outside the event horizon will not feel these effects. This event horizon plays a role in the emissions a black hole gives off. Though this seems impossible, it really can occur.
A supernova, however, does not always lead to the formation of a black hole. White dwarfs and neutron stars are the products of these explosions as well. A black hole is only created when the star is extremely massive and large; these stars are called super giants.
A black hole has one interesting and strange feature to it. It goes like this: two astronauts, A and B, are heading toward a black hole. One, A, has a watch and enters the event horizon. The other astronaut, B, sees that the watch of his companion slowly begins to stop ticking, and eventually stops all together, while astronaut A with the watch does not notice this effect. In other words, time, in astronaut B’s mind, stops for astronaut A.