Answer:
A black hole is a singularity, an incredibly large amount of mass reduced to a very small volume, essentially a single point. At that compression, the ordinary rules of atoms and motion cease to exist. Anything that approaches the black hole, even light, will eventually be pulled into that central singular point. The visible "width" of a black hole is where material falling into it radiates away energy from its increasing velocity. This is called the event horizon.
In terms of what it does, a black hole is very much like a hole : things fall in and don't come back out. If you think of space and time as a trampoline, a black hole would be like putting something that has unlimited weight on that trampoline. It would bulge the center of the trampoline completely out of view.