A black hole is basiclly a star whose gravity has become so strong that it has collapsed on itself and nothing, not even light can escape it. Einsteins realativity theory explains the basics of a black hole. The escape velocity needed for particles of light is lower than the amount of gravity being put on the light. Black holes and stars' gravity also cause time dialation. Mercury is close enough to the sun to experience this and i cant rememeber exacly how, but it has somthing to do with the rotation and the speed at which we view it orbiting the sun... SO if you were to look at a star and a black hole and a planet within the plain of the universe from the top it would look like this...




But when you throw in the dialation of time in space and you look at the grid from the side it looks like this....





Time and space becomes streched and distorted due to the high gravity, the larger the mass, the higher the affect.