There are science writers who explain black holes and the processes surrounding them pretty clearly, so you could try the library if you find no one here puts it quite succinctly enough, but I'll take a stab at this.
A black holes is just matter compressed so densely that its gravity won't permit even light to escape. In isolation nearly invisible and indetectable in the radio spectrum, a black hole is of course detectable due to its enormous gravity. Due to quantum effects, black holes *do* give off a slight amount of radio energy, called the Hawking radiation after Stephen Hawking who first proposed it. After long eons, hundreds of billions of years I think, a lone black hole will eventually evaporate to nothing due to this effect.
A black hole surrounded by matter, such as often happens in the centers of galaxies, has a voracious appetite. Matter is drawn relentlessly in, and the resulting maelstrom just outside the black hole's event horizon (the point where the black hole's gravity prevents even light from escaping) can give off enormous amounts of radiation. The black hole's enormous gravity is the ultimate cause of the maelstrom and therefore of the radiation, but the black hole itself is only contributing the virtually indetectable Hawking radiation.
--Percy
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