Technically, a black hole is a region of spacetime where, by nature of its great mass, gravity prevents anything from escaping: this includes light. The term ‘black hole’ was coined by Dr. John Archibald Wheeler during his work on gravitational collapse in 1967.
Earlier, in 1916, Karl Schwarzschild published the first modern characterization for a black hole based on Einstein’s theory of general relativity. His work gave rise to the well-known Schwarzschild radius. This is the radius from the center of an object, within which, if the mass of the object were compressed, its escape velocity would equal the speed of light. When the radius of an object is equal to or less than the Schwarzschild radius, it condenses to an infinite density and becomes a black hole. This radius is also called the ‘event horizon’.
If that did not make sense, consider this. If the radius of an object is equal to or greater that its Schwarzschild radius, it turns into a black hole. For the mass of the sun, this radius is about 2 miles (meaning that if I were to compress the mass of the sun into a sphere with a two-mile radius, it would turn into a black hole). For the Earth, this radius is about 9 millimeters (or about the size of a marble).
How are black holes formed? Currently scientists believe as a star of sufficient mass collapses its radius shrinks to the Schwarzschild radius. When this happens, the outer shell of the star (everything outside the Schwarzschild radius) explodes into space creating a supernova, whereas everything within that radius collapses into a singularity.
LOVE reading about these babies, btw, I linked to the article, I did not write this puppy :)