A black hole is one of the strangest objects in the universe, and while scientists do know much about them, there is much more scientists don't know about them. Stephen Hawking is a world authority on black holes.
Formation and development
Basically, a black hole is formed when certain types of stars (such as red giants and supergiants, both of which are several thousand times the mass of the sun when born) die. In the ensuing supernova, the star's core, which by that time is composed chiefly of carbon, oxygen, and similar elements, collapses in on itself. Normally, this forms a fast-rotating and incredibly dense radioactive star called a pulsar; if the core is crushed even beyond this stage, however, it becomes a black hole. At that point, the former star's gravity increases exponentially, to the point where even light is unable to escape it. The black hole then begins moving slowly through space, devouring anything that gets too close. Because even light (the fastest moving substance in the universe) cannot escape the black hole, crossing over its edge (referred to as the "event horizon") means that you can never get back out. Black holes continue to float through space for a time before they eventually explode, releasing a huge wave of energy many times stronger than a supernova (which itself is akin to many thousands of atomic bombs and can outshine entire galaxies). Stephen Hawking is a world authority on black holes.
Limits on knowledge
Beyond these facts, however, scientists know very little about black holes. It is widely believed that the center of a black hole is a zero-dimensional (meaning that it has no physical presence in any of the three known dimensions) point of infinite density, referred to as a singularity. This, however, is found unsatisfactory by many, and so an alternative possibility is being searched for.
Black hole technology
Miniature black holes represent the greatest chance humans have to build a warp drive, which is necessary to travel faster than light and so reach other galaxies within reasonable amounts of time.