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Albert Einstein's theory of general relativity explains the properties of black holes. The material inside a black hole is concentrated into a singularity: a single point of infinitely high density where space and time are infinitely distorted. Distant objects can escape from a black hole's gravitational pull, but objects inside the so-called event horizon inevitably fall towards the center (such objects would have to move faster than light to escape, which is impossible according to the laws of physics). The size of the event horizon and the distortions of the space and time surrounding it are determined by the mass and spin (rate of rotation) of the black hole. Space and time distortions cause unusual effects; e.g., a clock falling into a black hole will be perceived by a distant observer to become redder and to run slower.
Two types of black holes are found in the Universe: stellar-mass black holes and super-massive black holes. These are characterized by different masses and formation mechanisms.
A stellar-mass black hole forms when a heavy star collapses under its own weight in a supernova explosion. This happens after the nuclear fuel, which makes the star shine for millions of years, is exhausted. The resulting black hole is a little heavier than our Sun and has an event horizon a few miles across (for comparison, to turn the Earth into a black hole it would have to be squeezed into the size of a marble). The existence of such black holes has been inferred in cases where the black hole pulls gas of a companion star that orbits around it. The gas heats up as it falls towards the black hole and then produces X-rays that can be observed with Earth-orbiting satellites.
Super-massive black holes are found in the centers of galaxies that contain billions of stars. They may exist in most galaxies, and probably formed at the same time as galaxies themselves. They are millions or billions times as heavy as our Sun, as determined from the motions of stars and gas surrounding them. Spectacular activity can occur when gas falls onto the black hole (as observed in a few percent of all galaxies). Material is ejected in jets that emit radio waves, and heated gas produces X-ray emission. Observations of such X-rays may soon provide insight into the spin of black holes.
There are enough black holes in the Universe that there should occasionally be collisions between them. Such violent events send ripples through the space-time fabric of the Universe. Scientists are hoping to soon detect such "gravitational waves" for the first time.
Steven Hawking showed in 1974 that every black hole spontaneously and continuously loses a tiny fraction of its mass due to radiation. However, this Hawking radiation is negligible for the known black holes in the Universe, and will not be detectable in the foreseeable future.
Bibliography:
Begelman, Mitchell, and Martin Rees. Gravity's Fatal Atraction: Black Holes in The Universe. New York: Scientific American Library, 1996.
Couper, Heather, and Nigel Henbest. Black Holes. DK Publishing, 1996.
Thorne, Kip S.. Black Holes and Time Warps: Einstein's Outrageous
Legacy. New York: W.W. Norton & Company, 1995.
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