Fiber optics is a data communication technology which uses light to carry data over a communication link. The light is usually produced by a semiconductor laser now, and is carried over a special medium, optical fiber (the source of the name) to a detector at the far end, which converts it back into electrons.
Optical fiber consists of very clear glass, drawn out into a very thin fiber. Light introduced at one end effectively bounces off the walls of the fiber (which it meets at a shallow angle) as it travels down it. The fiber is actually former of concentric layers with a different index of refraction; as a photon traverses the layers, its path is bent around and it winds up reflected back into the body of the fiber.
Most wide area networks are now built out of fiber-optic links.