Shoutbox

One thing to dispute here: black holes are NOT objects.
In the loosest possible terms a black hole is a region of space that has so much mass concentrated in it that there is no way for a nearby object to escape its gravitational pull. Basically this mass concentration has an escape velocity greater than c, so not even light can escape. The idea of such a mass concentration that even light could not escape from originates with Laplace in the 18th century, then straight after Einstein's theory of special relativity Karl Schwarzschild managed some hypothetical solutions to the equations that described such an object. It wasn't until the 1930s that people thought "wow, these things might actually exist" - Oppenheimer, Volkoff, Snyder were the main researchers. They showed that when a sufficiently massive star runs out of fuel, it is unable to support itself against its own gravitational pull, and it should collapse into a black hole. That's as far as you can take it with special relativity, to advance you have to consider general relativity (the harder version).
In general relativity, gravity is a manifestation of the curvature of spacetime. Massive objects distort space and time, so that the usual rules of geometry don't apply anymore. Near a black hole, this distortion of space is extremely severe and causes black holes to have some very strange properties. In particular, a black hole has something called an 'event horizon.' This is a spherical surface that marks the boundary of the black hole. You can pass in through the horizon, but you can't get back out. In fact, once you've crossed the horizon, you're doomed to move inexorably closer and closer to the 'singularity' at the center of the black hole.
It is this singularity that is, in essence, the black hole. At the singularity our laws of physics no longer apply, the singularity cannot be considered an object at all. In conclusion a black hole consists of this singularity and the event horizon... a region of space. Not an object. Even the event horizon is not as well defined as you would think, due to Hawking's solution of the Black Hole Information paradox.
A note on what is emitted from a black hole: only one thing is emitted, Hawking radiation. The laws of quantum physics state (simplified :p) that the information emitted, when a black hole evaporates, about the matter inside the black hole can never be completely wiped out. Hawking's 1976 black hole model (whereby a black hole starts losing mass as soon as it has formed via radiating energy) showed that the data was totally destroyed, thus creating the paradox. Hawking's argument was that the intense gravitational fields of black holes somehow unravel the laws of quantum physics. In essence, Hawking's new black holes now never quite become the kind that gobble up everything. Instead, they keep emitting radiation for a long time, and eventually open up to reveal the information within. They also, unlike classic black holes, do not have a well-defined event horizon that hides everything within them from the outside world. Hence the nont-well-defined event horizon. This is the only emission from a black hole.
There are two indirect ways to detect black holes: x-ray binary systems and mass measurements on the centres of galaxies.

Edit: There is a further solution to black holes via string theory but it's hella complicated and I don't have the patience to explain it. :p