Currently, most people get their Internet, broadband or otherwise, through their phone or cable companies. For the most part, these services transmit data into your home as electrical signals over copper cable, although the vast networks that bring us data and telephone communication incorporate lots of types of wiring, including fiber optics. Fiber optic cables transmit pulses of light through glass (or sometimes plastic) strands, and are capable of transmitting data much more quickly than copper, especially over long distances.
Fiber optics have been in use in telecommunication networks since the 1970s, but the so-called "last mile" stretches from service lines into individual homes are usually still made up of copper wiring. Phone companies tend to use twisted pair copper cable (like traditional phone lines) to deliver DSL, and cable companies use the same coaxial cable through which they deliver cable TV. Most homes in the U.S. are already wired with at least one of these, if not both, and fiber into the home is fairly rare. But Google is looking to change this by laying thousands of miles of fiber cabling to create last-mile fiber networks.
Google Fiber networks include several basic components, although the actual design will likely vary by city. A fiber ring around the city feeds fiber into a fiber hut. Here the fiber lines are connected to devices that receive and transmit data from users' homes to the Internet and vice versa. From the hut, lines of fiber lead in various directions to telecom cabinets in neighborhoods, where the lines are further broken out into smaller bundles that lead to groups of homes. The fiber cables will run either above-ground on new or existing utility poles, or underground in conduit tubes, through neighborhoods, where individual strands of fiber optic cable will lead directly into homes, much like phone and cable lines do now.