A black hole is created when you have a very Heavy(mass) object concentrated in a very small area(volume), so a black hole is when nature has gone mad and density(mass/volume) has gone off the charts. So for example when a sun dies it shrinks to a very small dot and because suns are so heavy it becomes what we know as a black hole. That's the scientific understanding of black holes. Because Black holes are so heavy the gravity they emit is MASSIVE as gravity is determined how heavy an object is, a black hole's gravity sucks up anything nearby even LIGHT, it really is where nature has gone frcken crazy.
The black hole itself no one actually knows what it is but the common theory is because it's so dense it creates a tear in the dimension we live in, and since no one knows what happens when you enter a black hole another common theory here is that you enter another location in the universe. This a real theory about time/space manipulation.
If you want to know more about black holes because they are awesome, natures most destructive weapon. http://en.wikipedia.org/wiki/Black_holes
, but not all of this site is going to applicable to Naruto series as the concept is most important.