To understand what a black hole really is, we must first consider the subject of relativity. There are two flavors of relativity, special and general. Special relativity says that movement at a constant velocity in a straight line is equivalent to standing still. No matter how fast you travel, you MUST see light traveling away from you at the speed of light. So light must be traveling faster than you are.
Light is the fastest thing there is, and nothing else travels as fast as light.
Now let's apply this fact to black holes. The faster you throw the ball, the higher the ball will go. If you throw a ball fast enough, gravity will not be strong enough to pull it back to earth. You will have given it escape velocity. The escape velocity, obviously depends on the amount of gravity, which depends on mass and the distance.