A black hole is an object in space that is so massive that the escape velocity from its gravity, within a radius known as the event horizon, is greater than lightspeed. Since not even light can escape from a black hole, they can not be detected directly, but they can be detected by their effects on their surroundings. For instance, stars and other luminous objects may orbit around black holes. A star must be at least three solar masses for a black hole to form.
Black holes in Science Fiction
Black holes appear rather frequently in science fiction, and their properties often differ radically from what is known about real black holes. Most often, a black hole is actually one end of a wormhole, allowing ships to pass through them and emerge in another place or time (or both).