Currently, most of our data travels through either copper wire or fiber optic cable. Even when we send data via our cell phones over radio waves, which also travel at light speed, it ends up traversing the wired networks of the Internet at some point. The two most common types of copper wire for long-distance information transfer are twisted pair (used first for telephony, and later for dial-up Internet and DSL), and coaxial cable (used for cable TV initially, then Internet and phone). Coaxial cable is the faster of the two. But still faster is fiber optic cable. Rather than using copper to conduct data in the form of electrical signals, fiber optic cable moves data as pulses of light.
The "in a vacuum" reference on the previous page regarding the speed of light is important. Light through fiber optic is not as fast as light through a vacuum. Light, when moving through just about any medium, is slower than the universal constant we know as the speed of light. The difference is negligible through air, but light can be slowed down considerably through other media, including glass, which makes up the core of most fiber optic cabling. The refractive index of a medium is the speed of light in a vacuum divided by the speed of light in the medium. So if you know two of those numbers, you can calculate the other. The index of refraction of glass is around 1.5. If you divide the speed of light (approximately 300,000 kilometers, or 186,411 miles, per second) by this, you get around 200,000 kilometers (124,274 miles) per second, which is the approximate speed of light through glass. Some fiber optic cabling is made of plastic, which has an even higher refractive index, and therefore a lower speed.
Part of the reason for the decrease in speed is the dual nature of light. It has the attributes of both a particle and a wave. Light is actually made up of particles called photons, and they do not move in a straight line through the cabling. As the photons hit molecules of material, they bounce in various directions. Light refraction and absorption by the medium eventually lead to some energy and data loss. This is why a signal can't travel indefinitely and has to be boosted periodically to cover long distances. However, the slowing of light isn't all bad news. Some impurities are added to fiber optics to control the speed and aid in channeling the signal effectively.
Fiber optic cable is still far faster than copper wire, and isn't as susceptible to electromagnetic interference. Fiber can achieve speeds of hundreds of Gigabits per second, or even Terabits. Home Internet connections don't achieve those super high speeds, at least partially because wiring is being shared by many households over entire areas, and even networks that use fiber optics generally have copper running the last stretch into people's homes. But with fiber running all the way to your neighborhood or home, you can get something in the range of 50 to 100 Megabits per second of data transfer, compared to 1 to 6 Megabits per second from average DSL lines and 25 or so Megabits per second from cable. Actual data speeds vary greatly by location, provider and your chosen plan, of course.
There are also other things that cause signal latency (delay), such as the back and forth communication required when you access a Web page or download data (handshaking). Your computer and the server housing the data are communicating to make sure they are synchronized and data transfer is successful, causing a delay, albeit a brief and necessary one. The distance your data has to travel will also affect how long it takes to get there, and there could be additional bottlenecks at any hardware and cabling the data has to travel through to get to its destination. A system is only as fast as its slowest component, and every millisecond counts in the days of seemingly (but not really) instant communication.
There have been recent breakthroughs in transferring data over copper wiring at nearly fiber optic speeds via reducing interference and other techniques. And researchers are also working on transmitting data via light through the air, say using lightbulbs for WiFi, or transmitting laser beams from building to building. Again, light through the air does move at close to light speed, but nothing we have now is surpassing the speed limit. Can we achieve actual faster-than-light transfer?