What is a black hole?

I guess this is the first question you asked to yourself before reading this article.
Strictly, a black hole is a region of spacetime where there is a gravitational field so strong that neither light can scape from it. Let's understand this. Let's imagine we have a stretched sheet and we put a heavy ball in the middle of it. The sheet will deform, and if now you put a small ball in the sheet, this small one will be attracted to the big one. This is a simple explanation of how gravity works. Now, let's imagine that the spacetime is a kind of stretched sheet, n (w if we "put a black hole" on it, the black hole will deform the sheet in a way that every single objet (or energy!) near it, is going to be attracted (and also, swallowed!). So... congratulations, now you know (more or less) what it is a black hole

How are black holes created?

And... what if a told you that... black holes were stars so many years ago? Well, let me explain this to you.
In the core of the stars, hydrogen fuses to make helium. And to make things easier, we are going to say that when that reaction can't be made, the star dies. What happens when a star dies? Well, when this happens, the star begin to collapse under its own gravity (this did not occur before because the energy released in the fusion of hydrogen was avoiding it) and depending of the mass of the star, this collapse can end in white dwarfs stars, neutron stars or black holes. Amazing, isn't it?

So, let's enumerate what we know:
-Black holes are so massive that they deform spacetime in a spectacular way.
-Black holes born when a very very very massive star dies.
Now, let's learn other things:

What types of black holes are there?

We can differentiate black holes depending of the mass. (Here, Ms=Mass of the Sun=1.98x10^30 kg =1980000000000000000000000000000 kg)

Class of BH	Mass (Approx.)
Micro Black Hole	<<<Ms
Stellar Black Hole	10 Ms
Intermediate mass Black Hole	1000 Ms
Supermassive Black Hole	Billions of Ms

Have we ever discovered a black hole?

In 1995, some investigators demonstrated the possibility existence of supermassive black holes in the middle of some galaxies, after making some calculations it was verified that something was deforming the path of the light emitted from the center of The Milky Way (our galaxy!!). What could it be?... Exactly, a black hole, concretely a supermassive black hole called Saggitarius A*.

Finally, let's make a second count of what we know:
-Black holes are so massive that they deform spacetime in a spectacular way.
-Black holes born when a very very very massive star dies.
-We classify black holes by their mass.
-There is a supermassive black hole in the middle of our own galaxy.

Well, here we finish our trip through black holes. This was like a 'Black holes 101' or something like that. I really hope you enjoyed this article and I would be so happy if you're now more interested in this topic or in science in general.
Thank you for reading, see you soon!
Regards!

References

Images from Google Images. Link: https://phys.org/news/2015-10-black-hole-paradox.html
Wikipedia:
https://es.wikipedia.org/wiki/Agujero_negro#Los_agujeros_negros_en_la_f%C3%ADsica_actual