Black holes were originally 'discovered' to explain what happened to a star of greater than about 3 solar masses, when it ran out of fuel, and collapsed upon itself. But observational astronomy has found evidence that lurking at the centre of most, if not all (including our own) galaxies lies a super-massive black hole with a mass or millions of even billions of solar masses. Super-massive black holes provide the only credible source of energy to power a quasar.

These objects are also interesting from a theoretical point of view also, as the more massive the hole, the larger the event horizon, and the less the tidal forces you experience when you cross it. With an ordinary black hole the difference in gravity experienced by your feet and you head, and the warpage of space crushing your feet inwards (as shown by relativity) would turn you into spaghetti, long before you reached the event horizon. However with a massive hole this is much less, and you could cross the event horizon unharmed.

For a hole 15 trillion times the mass of our sun, its event horizon would be 29 light years in circumference, and you could use a rocket exerting 10 g's of thrust to hover only just above it, at 1.0001 circumferences. 1Entire stars could dissapear into it, without getting disrupted; the hole would be truly black.....

If the universe does not collapse in big crunch and carries on expanding, then it may be that the Hawking radiation around such giants will be the only souce of energy left in the universe, far down the stream of time, when all the stars have died...

1The question has been asked by rootbeer277 'Circumference is the measure around the circle, wouldn't this be better expressed in radius or diameters?' The reason for this is that geometry is tricky when you're talking about black holes; in fact space/time geometry involves whole branches of tricky maths, you may remember something about 'rubber sheets and cannon balls'. Check out the whole general relativity thang! The gravity of the black hole will distort space/time so severly, its radius will become (possibly, wait 'till quantum gravity is solved) infinite. Really a black hole (as described by current theory) is in itself a whole mathmatical abstraction, and could just defined as a region of space where gravity is so strong, light can pass in, but not out of.