The flash animation introduction to this page gives an easy to understand definition of a black hole. The site describes itself this way:
"Black Holes — Gravity's Relentless Pull is an exciting, in-depth exploration of the astronomy and physics of black holes. The site was created in a collaboration between astronomy, education and Web professionals. It offers astronomical images, animations, interactive experiments (some with audio), and an encyclopedia of accurate, up-to-date information.
Black holes are probably the most extreme objects in the Universe. The site shows that even the most mysterious of things can be understood through scientific inquiry. It also explains many basic concepts about physics, astronomy, and science in general, including light, gravity, and the scale of our Universe. Exploration of the site is sure to fascinate and to instill an interest in science. No specific prior knowledge about astronomy or physics is required."
The home page of the Hubble site is also worth exploring. The Black Hole page can be found through the "Explore Astronomy" section.