The black hole is a dense mass which produces such a strong gravitation that not even the light can escape. The three basic forces of physics cannot counteract this either, which is why the entire mass collapses and forms a physical singularity in the centre of the black hole.
A binary star consists of two stars that are so close to each other in the sky that only a small angular distance is visible between them from Earth. With a very high probability, they are gravitatively coupled and orbit around a common center of gravity.
In contrast to the globular clusters, an open star cluster contains only a small number of related stars. These twenty to several thousand stars are formed from the same giant molecular cloud and have a lower star density.