Black hole is a region of space in which the gravitational field is so powerful that nothing can escape after falling into it. Even electromagnetic radiation light for example - cannot escape the pull. A black hole can be detected when it interacts with matter outside the event horizon, the boundary of black hole. Einstein's Theory of General Relativity postulated that a large amount of mass is present within a small space and all parts through space are pulled inward toward the centre of the mass, forcing all matter and radiation to fall inward. Proponents of quantum mechanics, however, believe that a black hole does not hold the matter forever but slowly leaks it in the form of thermal energy. Black holes of different sizes are classified as super massive, intermediate mass, stellar mass or micro black holes. NASA GLAST satellite had to search for micro black holes as one of its tasks. Black holes are also classified on the basis of their angular momentum and electric charge. Black holes rotate because the stars they are formed from rotate. They mostly spin very rapidly because they retain the angular momentum of the star they were formed from. But there are a few non-rotating black holes also. The effects of a charged rotating black hole are more complicated.