A black hole is basically described as a big thing that nothing can escape. But seriously, they get much more interesting..
How does a star become a black hole?
They start off as Regular Stars, that shine because of light that is radiated from the core. Then, as the star begins to die it becomes a Superdense Body, whose gravity is so strong it can actually pull light back. Once it becomes a Black Hole the core has become so dense, that no light can escape it. The star would have to be 30 times the size of the sun, basically, and it dies to collapse under it's own weight.
What happens then? It shrinks to an infinatly dense point known as a singularity. Around this is an imaginary circle known as the event horizon beyond which all light from the object is turned back by the force of it's own gravity.Einstien proved that gravity can capture light, and distort time and space. Inside the event horizon, the entire concept of space and time breaks down.
Whoa! Really? So..what happens if you get sucked in?
Ah..that's always a fun trip. First, you would enter the event horizon, and from then on there's no escape possible. Say you fell in feet first. Well, the gravitational pull would be so great on your feet compared to your head that you would instantly be stretched like spaghetti. Now that you're thinner than all the models on Earth, you of course want to gloat back at them. Unfortunatly, if you looked back as you fell, you would see the future history flashing before your eyes, and not be able to communicate to anyone. As you fall closer and closer to singularity, your spaghetti body will be torn apart, atom by atom by atom. (Remember about 10 million atoms stretch across the tip of your pinkie finger. This really hurts.) And finally, at the end of this journey, you reach singularity, where everything known about the universe becomes null and void. And a person outside couldn't watch your demise, as pleasent a torturing device as this sounds. As they fell in the event horizon, time would slow so much, it would get to a point where it took infinately long to reach it. Also, since no light escapes, you wouldn't see them become spaghetti.Fun trip, huh?
Hardly!! Are we in any danger of these things??
Nah. The closest stellar size hole is at least 15 light years away, and the supermassive ones are confined to the centers of distant galaxies. Theory states there may be mini black holes scattered around, remnants of the Big Bang, but none have been found.