Black Holes are places in which there is extreme gravity and even light can not escape from. It can be specified that it is deformed space time according to general relativity theory. They represent the collapse of a massive star. When the massive star is completely dead its mass deform the spacetime and form a balckhole.
Event horizon (Schwarzchild Radius) is the last place that the light or other objects can escape from blackholes, beyond the horizon any material includign the light will be lost and lose their formations. Interestingly a blackhole obeys all laws of gravity. Quantum mechanism or quantum gravity is trying to describe the behavior of the material inside the blackhole because a blackhole collects all material into a single point in the center, called central singularity .
After the solution of this problem of behaviour of materials inside the blackhole, everything about blackholes will be more understandable. In the space there are blackholes sizing from 12 kilometres to the size of solar system. There can be smaller or larger ones also. There is also one big black hole in our galaxy Milky Way.