Optical fiber - more commonly referred to as fiber-optic - is a hair thin artificial fiber, which is capable of providing higher data transmission standards, when compared to copper based communication networks.
Simple put, fiber-optic communication works by transporting light, and it achieves this by using "total internal reflection". Total internal reflection is an optical phenomenon where light bounces when directed at a specific angle. Fiber-optic cable is hair thin, made from glass, and is capable of supporting total internal reflection.
As the light bounces, it can be transported great distances; without the light current suffering as much attenuation (a gradual lowering of intensity) - when compared to electrical cable systems, such as BT's aging copper landline network.
Fiber-optic, surprisingly, is not a modern discovery: using reflected light as a means of communications was demonstrated in the 1800's. The issue was finding a reliable way of implementing the technology. The current fiber-optic cable was invented in the 1970's.
Fiber-optic broadband networks are slowly being launched across the United Kingdom; which is sometimes referred to as super-fast broadband. Fiber-optic, due to it's lower attenuation, can provide vastly superior download and upload speeds when compared to copper and mobile broadband networks.