They're not your ordinary black holes. They're bigger. They're badder. They are supermassive black holes capable of producing the largest eruptions since the Big Bang. But, despite their massive size, we're just starting to understand these forces operating in the center of galaxies sometimes billions of light years from Earth.
The documentary above (running about 18 minutes) offers a reasonably good primer on supermassive black holes. Or, to get another angle on things, you can turn to Frontiers and Controversies in Astrophysics, a course taught by Charles Bailyn at Yale. Lecture 15 (watch here) is specifically dedicated to these mother-of-all black holes.