A black hole is a single point of infinite density, with an infinitely high curvature of space and time, infinitely small volume, and infinitely high temperature, as well as the surrounding empty region of space, inside the range of which is unobservable. According to Albert Einstein’s Theory of Relativity, when a dying star implodes, it collapses into a point which becomes a black hole. This black hole swallows all nearby light and matter in that region of the universe.
The formation process of a black hole is similar to that of a neutron star: when a star is about to die, its centre rapidly shrinks under the weight of its own gravity, and collapses with a powerful explosion. When all of the matter in the core becomes neutrons, the shrinking process stops, and the the matter is compressed into a dense celestial body, while the space and time inside it are also compressed. However, in a black hole, the mass of the star’s nucleus is large enough that the contraction process cannot be stopped, even by the repelling force between the neutrons. The neutrons themselves are squeezed and crushed to a fine dust under their own gravitational force, the remnant being an unimaginably dense body of matter. Because of the gravitational force created by the mass, any nearby object is absorbed by the matter. 
Explained simply, to begin with a star usually only contains hydrogen, the hydrogen nuclei momentarily collide, causing fusion. Because of the star’s large mass, the power created by the fusion is countered by the star’s gravity, and the star’s structure remains stable. From the fusion of hydrogen nuclei, a new element is created: helium. In turn, helium nuclei also undergo fusion, change their structure, and create lithium. Following this principle, new elements are created according to the order of the periodic table: beryllium, boron, carbon, nitrogen, etc., up to the creation of iron, when the star collapses. This is because iron is a relatively stable element, and the power it releases from fusion too small, and so fusion stops, and iron remains inside the star. As a result, the star’s interior does not have enough energy to contend with gravity from the star’s enormous mass, thus triggering the star’s collapse and ultimately the creation of a black hole. It is called a ‘black’ hole, because the force it generates does not allow surrounding light to escape. Like neutron stars, black holes develop due to a solar mass tens or even hundreds times greater than that which arises from the star’s development.
When a star ages, its thermonuclear reactions deplete the star’s fuel, while the energy produced by the centre is already very little. Thus, its force is not sufficient to support the enormous weight of the star’s outer layers. Therefore, the core begins to collapse under the gravity of the outer layers, and the material cannot be stopped from advancing towards the centre until a body is formed with an almost infinitely small volume and almost infinitely large mass. Should the radius contract to a certain size (smaller than the Schwarzchild radius), the curvature of spacetime caused by the mass means that even light cannot be emitted, and a black hole is born.