Normally we think that a black hole is an enormous amount of matter even light cannot escape it. This idea is very crude, consider our galaxy, the milky way has enormous mass but it is not a black hole because we get light from other galaxies like Andromeda.
For a black hole, we need only slightly more than ?2 Solar Mass (M?).A black hole is a trapped surface of spacetime exhibiting a strong gravitational pull such that no light or particle can escape from it (That is, gravity is so strong that the escape velocity is faster than light). Black hole is believed to form when stars die and their massive bulk collapses inward, creating intense gravitational fields. No one has “seen” a black hole, but scientists have observed gravitational evidence consistent with predictions about them that is it has to be observed only indirectly, so most scientists believe they exist.
Black holes are one of the most famous predictions of Einstein’s theory of general relativity. They are trapped regions of space, characterised by electric charge, mass and rate of rotation, from which nothing can escape. Supermassive black holes exist at the centre of probably every galaxy, with a mass ten to one hundred billion times greater than the sun.The black hole is more properly defined and recognized by the event horizon that hides the supposed singularity.
At the horizon, the metric changes sign and things beyond are not causally connected to us. Thus while discussing black holes we have to begin with the most advance conclusions from special relativity and corresponding terminology. To describe both mathematics as well as physics we need the terminology of differential geometry.
Application of string theory in studying black holes is a most significant piece of evidence which favors string theory.After the formation of a black hole by Stellar collapse, the laws of black hole physics start operating. Like a black hole thermodynamics.