science books tell us that black holes are real
If then , where is it and what actually is it?

Yes, there is very strong scientific evidence that Black Holes exist. They are hard to detect because their gravitational field is so strong that light cannot escape. This is why they appear black. Good evidence can be found for a black hole if it happens to be near a companion star.
One can see matter being drawn off the companion and large amounts of radiation being created.
There is no ONE BLACK HOLE. It would be impossible to know how many black holes actually exist because they are hard to detect. There is also the fact that they are being created and according to Hawking, evaporating.

There is probably a Black Hole at the centre of our Galaxy and there is another one in the vicinity of the Large Magellanic Cloud

TUT317 is spot-on.

Black holes happen to occur when a star - a star much bigger than, say, our own sun - collapses. It collapses because the gravitational force is stronger than the weakening counterbalancing energy/nuclear output, which causes an implosion of immense proportions.

See, stars have only enough energy to last for a limited period, sometimes only hundreds of millions of years. After the star has expended all of its energy, it implodes in on itself.

But here is a really hard concept to grasp.
There is evidence suggesting that there are decillions of black holes on Earth right now.
Luckilly, they are reallly, really small.

I have to admit I'm not aware of that theory. If they are really tiny then they would probably evaporate very quickly.

Have you got any more info on tiny black holes? I do remember something about the Large Hadron Collider and the possibility of it creating tiny black holes

This theory is still being explored, even today.
I would either Google or watch the history channel.