Image obtained from http://web.mit.edu/kayla/Public/Backgrounds/
A black hole is an intense gravitational field in which not even light can escape. Most common black holes, known as stellar black holes, are caused by dying stars. The only stars that are capable of producing this are stars that have a mass of 20 times greater than the mass of our Sun or more.
What happens is the nuclear energy within the star starts to die out. Then gravity takes over. The star then implodes causing a black hole.
Black holes have been theorized to allow time travel since the gravity is strong enough to bend space-time. But you are more likely to get crushed before that ever happened.