Black holes are perhaps the most fascinating and mysterious areas of space that humans have thus far discovered. We imagine them as big, hungry beasts that eat up space and time. While this is conceptually accurate to a degree, black holes are far more complicated. They are actually collapsing space and trapping everything, including light, within themselves.
Formation of black holes
The only currently detectable manner of black hole formation is gravitational collapse. Stars are, so far, the only observed stellar objects with enough mass to cause such an event. Put simply, when the inside of a star can no longer support its own gravity, the star collapses on itself and creates a black hole.
In the early stages of the universe, it is thought that there were a number of stars big enough to collapse and create massive black holes. Black holes continue to grow as they take in essentially everything near them, so these are possibly the same black holes that are now supermassive black holes. These supermassive black holes are hypothetically located at the center of most galaxies, including our own.
There are potentially other ways for black holes to form, though none has been observed. It has been theorized that a massive collision with enormous amounts of energy could produce enough density for a black hole to appear. The CERN Super Collider is an attempt to mimic such collisions.
Radiation and black holes
It is one of the best-known and most interesting facts about black holes that even light cannot escape them. However, they are not without emissions of their own. Stephen Hawking was able to demonstrate that black holes emit radiation. This is now known as Hawking radiation.
This discovery by Hawking and corroboration by other scientists indicates that black holes do not grow indefinitely. If they emit radiation, they will eventually evaporate because they are losing particles. This is truer for small black holes than large ones, as the larger the black hole, the smaller the amount of radiation emitted.
Event horizon and beyond
The event horizon is an almost scary invisible boundary that is often used in science fiction. This is essentially the limit in space-time around a black hole from which, after crossing, nothing can escape. In other words, whatever crosses that frontier is collapsing into the black hole without question.
Ergosphere and staying still
A very neat feature of rotating black holes is called the ergosphere. The rotation of the black hole will pull objects in the surrounding ergosphere into the same rotation, according to the principles of general relativity. Therefore, nothing in this area of space-time can stay still. In fact, anything near the event horizon of such a black hole would have to move faster than the speed of light in the opposite direction of the black hole's rotation in order to stand still.
How do we know black holes are there?
Black holes are not traditionally observable objects. In other words, you cannot just look and see them. You see them through their actions. Their existence is inferred by what they do to the things around them.