A black hole is a star in which the pressure forces can not balance the forces of gravitation anymore. A black hole is what remains of a star that have collapsed under its own weight, crushing all its internal material to a state of infinite density.
Nothing is left after the star has collapsed expect an extremely intense gravitational field.
So, if the mass of the star is so large and the radius so small such that this velocity equals the speed of light, then light will not be able to escape and the body will appear black. This cirtical radius is called the Schwarzchild radius. Near and inside a black hole, the curvature is extreme. At the center of this circle, the surface has a kink called a singularity. There, the distortion of space and time is infinite.