This is the point of no return. If light, gets closer than this point it is dragged down to the surface, it cannot escape - other matter is trapped much further out. This means that as anything approaches this point the energy released by it, as the gravitational force of the Black Hole tears it apart, is able to be radiated out into space like a last desperate cry for help. Only that part of the energy that is radiated away from the Black Hole is able to escape, so we get the appearance of a large luminous ball surrounding the Black Hole - the Event Horizon. It is this sphere that we measure as the size of a black hole. The Black Hole itself is believed to be a singularity (a point in space with no size - think of it as a grain of sand if you like). Often, because of the spin of the Black Hole and because of the effect of its massive gravitational pull on electromagnetic radiation, it will develop an outpouring of energy along the axis of its spin and may even develop a kind of luminous disk of trapped matter around its equator.