What is a black hole?

BLACK HOLES ARE points in space that are so dense(closely compacted in substance) they create deep gravity sinks. Beyond a certain region, not even light can escape the powerful tug(pull (something) hard or suddenly) of a black hole's gravity. And anything that ventures(a risky or daring journey or undertaking) too close—be it star, planet, or spacecraft—will be stretched and compressed like putty in a theoretical process aptly (aptly describes his hard work to complete the work) known as spaghettification. 

spaghettification:-

the process by which (in some theories) an object would be stretched and ripped apart by gravitational forces on falling into a black hole.

There are 4 types of black holes: stellar, intermediate, supermassive, and miniature. The most commonly known way a black hole forms is by stellar death. As stars reach the ends of their lives, most will inflate, lose mass, and then cool to form white dwarfs. But the largest of these fiery bodies, those at least 10 to 20 times as massive as our own sun, are destined to become either super-dense neutron stars or so-called stellar-mass black holes.

In their final stages, enormous stars go out with a bang in massive explosions known as supernovae. Such a burst flings star matter out into space but leaves behind the stellar core. While the star was alive, nuclear fusion created a constant outward push that balanced the inward pull of gravity from the star's own mass. In the stellar remnants of a supernova, however, there are no longer forces to oppose that gravity, so the star core begins to collapse in on itself.

In their final stages, enormous stars go out with a bang in massive explosions known as supernovae. Such a burst flings star matter out into space but leaves behind the stellar core. While the star was alive, nuclear fusion created a constant outward push that balanced the inward pull of gravity from the star's own mass. In the stellar remnants of a supernova, however, there are no longer forces to oppose that gravity, so the star core begins to collapse in on itself.

If its mass collapses into an infinitely small point, a black hole is born. Packing all of that bulk—many times the mass of our own sun—into such a tiny point gives black holes their powerful gravitational pull. Thousands of these stellar-mass black holes may lurk within our own Milky Way galaxy.

source:-nationalgeographic