Black holes occur commonly in science fiction, and are widely regarded as something of a mythical construct bordering on pseudoscience. Black holes, however, are quite scientifically motivated and established, but have only some of the properties science fiction seems wont to employ them for. A black hole can be defined as an object whose escape velocity exceeds the speed of light.
Nothing can travel faster than light, and so nothing can escape a black hole.
An object from which no light nor anything else can ever escape is indeed black, and something of a hole. One can turn this equation around to find the radius, for a given mass, at which the escpe velocity equals the speed of light, c . This radius is called the Schwarzschild Radius . Any object whose radius is smaller than this is a black hole. A curiosity is that, since mass increases as the cube of radius, but linearly with the Schwarzschild radius, very large black holes can have very low average density.