What? What's with that bored look? Oh, allright, I'll explain.
A black hole is, of course, the naturally-occuring infinititely-compressed remains of a star that has, at a minimum, five stellar masses. Now read what I just wrote again.
Infinititely-compressed. And it occurs naturally.
Because of that compression, they have a massive gravity well, known to be powerful enough to not even let photons escape. It should be mentioned that photons have no intrinsic mass. Thats why they move at the speed of light.
Black holes are powerful enough that something that literally has no weight can't escape.
Now, anyone remotely familiar with physics can tell you that Quantum Physics and General Relativity aren't compatable. The equations break down when you try to use one in the other's field of study. Fortunately, 99% of the time, the two never need to meet for modelling purposes. You see where this is going.
Black holes are so powerful that our understanding of math breaks down when we try to fully model one.
And, of course, when two black holes meet, they merge and become one "larger" black hole. Oh, and it's belived that there are about 100 million of them in our galaxy.
Yeah, we all know what they are and are generally ho-hum about 'em, but I don't think most of us appreciate how strange and terrifying they actually are. At least, without knowing the specifics.