About Black Holes

To put simply black holes are most massive lumps squeezed into really small space. They are the structures which could not bear the load of its own mass and collapsed, these are very dense,  if we take a teaspoon matter from the black hole it will be more massive than Earth.
There is a force called gravitational force which is greater towards the body the massive it is. So the gravitational pull of the black holes are very strong, so strong that if you try to enter it you will be pulled inside in the form of spaghetti, not in one piece.
How they are formed?
Well! they are formed from the massive stars, when a big star runs out of fuel it cannot bear the pressure of its matter and matter is collapsed to the center, as more matter is collapsed the center becomes more massive, and the more massive it becomes it attracts more matter inside. So it is the drastic effect of gravitation that makes the black holes.
According to one theory called general theory of relativity gravity also has some effects on light. Light bends towards the massive objects.
In case of black holes light that goes in never comes back because it is trapped within the gravitational pull of the black hole.
We see the objects as the light imposed on it is reflected, but in case of black holes the light is never reflected.
Then how we see the black holes?
We cannot see them (actually we can see trace of radiation emitted from it.). But we can feel them. When light from distant stars is seen to be bending towards a region (seeming empty) without any apparent cause scientists draws the conclusion that there is a black hole.
But black holes are not completely black as they emits some amount of radiation called Hawking Radiation, named after the great scientist Stephen Hawking who proved that black holes must withdraw some radiation.
No one can know what goes on inside a black hole.
To know anything about any object we impose light on it and as some amount of light reflects and absorbed. But if you shed light on the black hole it will be all absorbed , never to come back.