What is black hole? how black holes form?

Black Hole is a Region of space time having very powerful gravitational force that nothing can stop even particles,electromagnetic radiation such as light can escape from inside it
Albert Einstein first predicted black hole in 1916 with his general theory of relativity . The name "Black-hole" was given by an American astronomer in 1967 & was first discovered in 1971 there are 3 types of black hole:-
1)Stellar black-holes
2)Supermassive Black-holes
3)Intermediate Black-holes

So, Now how do black hole forms?

BlackHoles are thought to have formed in the early universe soon after the bigbang when the big stars collapse itself black holes are formed sometimes collapse also forms supernova that blast the star parts into space
We cannot make black holes in laboratory. Scientists think that the smallest black holes formed when the universe begain. Stellar black holes are made when the centre of a very big star falls in upon itself or collapse & when this happen it causes a supernova, Now A supernova is a expoding star that explode part of star into spaces

How do we see blackholes?

According to NCSA ( National center For Supercomputing Applications )we cannot see a blackhole itself ( because even light cannot escape the blackhole's gravitational field ) but we can see blackhole effect on nearby matters

What is stellar mass blackhole?

It is a blackhole formed by the gravitational collapse of a massive star there masses ranging from about 5 to many tens of solar masses these are also called collapsars . New observations with NASA's Chandra X-ray Obsevatory have clocked the fastest wind ever seen blowing off a disc around this stellar-mass black-holeThe record breaking wind is moving about twenty million miles/hr or about 3% the speed of light this is nearly ten times faster than had ever been seen from a stellar-mass black-hole

Do all stars become black-holes?

Nope, only stars with very large masses can become blackhole it should be about 10 to 20 solar mass for the star to become a black hole although, the estimated mass of the black -hole is 2 to 3 solar masses

What is Intermediate-mass Black-Hole?

An intermediatte-mass black hole is a hypothetical class of black-hole with mass in the range 100 to one million solar masses significantly more than stellar black-holes but less than supermassive black-hole. The strongest evidence for intermediate-mass black-hole comes from a few low-luminosity

Could A black hole Destroy Earth?

No a black hole can't destry earth because no black hole is close enough to the solar system to the earth if a black hole enters our solar system it is more likely that our earth will get swallowed by it at a distance 100,000 times greater than that between earth & the sun the black hole would pass through the oort cloud it's possible that this would send a large number of comets towards earth & to the other planets as the black-hole enters our solar system it would be invisible to us aside from a slight gravitational lensing effect on distant star it's not until it started to tear gas from the outer gas giant planets that a noticeable accretion disc then a region of heated dust & gas will sorround the black-hole then only the super power of black hole is visible to us

Now, So many people thinks what happen inside a black-hole?

The answer is no one knows ;)
We basically understand what happens outside the blackhole as you approach its event horizon thath infamous point of no return The event horizon is where the escape speed exceeds the speed of light you'd have to be going faster than light which is practically impossible to escape from the the black hole.
According to physicists, with a black hole there is something called singularity. A singularity is what all matters in a black hole gets crushed into. Some people says that inside the black-hole there is a point of infinite but that's totally wrong. Others say that the singularity is actually a whole surface inside the event horizon. We don't know in although in real black-holes singularities doesn't even exist maybe.

The Conclusion

Black-holes are full of mystery & wonders there are also something interesting in their characteristics still, scientists are continously researching about these amazing things even we know that they are invisible there are many conclusion but they may never be a conclusion to black-hole.

References

1)-https://en.wikipedia.org/wiki/Black_hole
2)-https://www.nasa.gov/audience/forstudents/5-8/features/nasa-knows/what-is-a-black-hole-58.html