By Mohan Babu
The best way to answer this question is by saying what it is not first. Black hole is certainly not a hole in the space. It is a region in space with a heavy concentration of matter. It can contain million times the mass of our Sun and so has enormous gravitational pull. The gravity is so strong that nothing including light can escape its pull. Since light cannot escape a black hole, it would appear black. Each galaxy is believed to contain a super massive black hole in its center. Our own milky way galaxy has one in the center and the spiral arm of our galaxy stretches outward from this super massive black hole. So black hole represents a very high density of matter but it is certainly not a hole in space.
The size of a black hole is a matter of ongoing research. Not much is known about the minimum or maximum size a black hole can have. People expect that the size can be as low as our Moon or even less. Most agree that the black holes are not exactly points in space but they do occupy a certain volume.
Since we cannot see a black hole, the only way we can infer its presence is by understanding the gravitational influence on nearby objects (stars). There is also one more effect that helps us infer its presence. It bends the light from distant sources around it and tends to enlarge or distort the background radiation. This is called gravitational lensing. This concept has been confirmed in observations.
There were also discussions about creating black holes of smaller size in a laboratory. It is debatable whether such black holes would be similar to the natural ones in the absence of a gigantic gravitational pull. The gravitational effect of any normal object on the surface of the Earth would be negligible compared to our planet's gravity. The present efforts of creating black holes in a laboratory are restricted to trapping light but not matter.
The black hole have an outer boundary called an event horizon. This is the point of no return for any matter or light. This is a subject of ongoing research where there is still some debate on whether black holes would have an event horizon or not. So it is enough if we know that the black holes are regions in space with a heavy concentration of matter and intense gravitational pull. Science fiction authors often rely on black holes and worm holes to explain time travel. I am not sure if time travel is possible in a black hole. But I am certain that a black hole's intense gravitational pull will rip me apart if I dare to go closer.