What is a black hole.....?

Question

...and when stuff gets sucked into it where does it go?

Answer 1

This is actually a question that is very difficult to answer simply because we really don't know for sure what is inside a black hole. We have theories and models that seem to predict all the affects that black holes have, but since there is no way to see inside and nothing really comes out, then we may never have a definiative answer.
That being said, however, this is what we think...
A black hole is a the result of the death of a very massive star. While it was burning, the energy created within its core had an outward force which opposed the gravitational collapse of the star due to its mass. When the fuel inside the star ran out, the mass crushed the star. There are some very spectacular results of that crushing which include supernovas and neutron stars (pulsars) but that is something best left for another question.
The star will be crushed down to the point that the electrons are drawn down into the atomic nuclei and the protons and electrons are crushed together to form neutrons, but it doesn't stop there (that is where neutron stars stop). The gravity is so powerful that it crushes the neutrons down into what is essentially nothing. We say that the star is crushed down into a singularity. That means it is crushed down to a point that has no length width or height. This is a theory only at this point, but the laws of quantum physics suggests that it is true. To the best of our knowledge, there is no force in the universe that can withstand the magnitude of that much gravity.
At some point, the crushing of the star makes it so compact that the escape velocity (the speed in which you must travel to get away from the star's gravitational pull) exceeds that of the speed of light. When that happens, the star appears to "wink" out of existance. What has actually happened is that the light leaving the star simply is trapped by gravity and so it just appears to go dark.
The distance whereby which light is trapped is called the Event Horizon.
Inside the Event Horizon, and anything that wanders close enough to drawn inside will be drawn down into the singularity and crushed out of existance.
There are some theories that black holes are truely holes that lead into other universes, but that only energy can pass through them.
At this point everything becomes speculation and you guess is just as good as anyone elses.