An event horizon can basically be thought of as the boundary of a black hole, although there's nothing actually there that could be touched or anything like that. What's special about an event horizon is that once anything, matter or even light, passes beyond the event horizon, it can never escape from the black hole, and will continue falling towards the center of the black hole, which is called a singularity. This is because at that point, the gravitational field of the black hole is so strong that nothing, not even light, can reach the escape velocity necessary to leave the black hole. Because of this, it's impossible to get any information out of a black hole, so we can't really know what's beyond an event horizon.

However, there are a few things we can tell about black holes by studying General Relativity. If you were to watch something fall into a black hole, you would see the object fall slower and slower, and you'd never see them reach the event horizon. This is because the light that the object is emitting is having a tougher and tougher time escaping the black hole's gravity. And since light cannot escape from inside an event horizon, you'd never be able to see a signal that the object fell past it.

But if you were to fall into a black hole, and were looking out, you'd see the opposite. Outside light from the rest of the universe would be falling in towards the black hole faster and faster, from your perspective, and you would see all of the universe play out in front of you. This is a peculiarity of the warping of space and time from the black hole. The moment you cross the event horizon wouldn't appear to be anything special to you, but you'd never be able to escape back into the rest of the universe to describe what you saw. This is what the mathematics tells us happens, but even if someone were curious enough to want to see this for themselves, they wouldn't survive very long...

As for what's actually beyond the event horizon, at the center of the black hole would be a singularity. This is the point were all of the matter and energy that falls into the black hole falls into. Since it's infinitesimally small, you wouldn't be able to see it, even if light were able to escape. As you fall closer and closer to the singularity, the forces of gravity pulling on you would get stronger and stronger. However, it would be much stronger at your feet than your head (assuming you're falling feet first), since they're closer to the singularity, so the gravity would start to rip you apart. Eventually all of the matter of whatever reaches the black hole gets pulled into the singularity, which only takes up one infinitesimally small point in space.

What happens beyond that isn't really known - in fact it's impossible for us to know given our current theories. Some people may conjecture that the matter gets emitted in a "white hole" somewhere else in the universe, or in a different universe, but this is most likely science fiction. No evidence has been seen for white holes anywhere, and in addition, all the matter that gets pulled in is ripped apart and collapses into one point, so it probably wouldn't be recognizable as what it once was if it ever were able to escape.

That depends on whether we are talking about the perspective of the object falling through the event horizon, or of an outside observer.

The event horizon is the distance from a black hole in which space-time closes, that is, all possible lines of motion still lead to the singularity at the very center of the hole. The wavelength of anything trying to get out is redshifted into infinity, meaning that even light will to continue to fall inward. Thus, it is the point of no return; as far as the outside observer is concerned, any object that falls through the event horizon will see the object disappear, because light emitted by or reflecting off of the object can no longer escape out to where the observer can see it.

The object falling through, from its own perspective, however, notices nothing. There is no local measurement that you can make that will tell you if you are inside of an event horizon. You will eventually fall into the singularity, though, a point where space and time come to an end. Because the gravitational tidal forces that result from falling into a singularity become infinite as the singularity is reached, even particles as small and strongly held together as protons or neutrons will still be ripped apart by the tides as they actually reach the singularity, however.