Most habitual PC users have a nest of cables behind their computers. You have your keyboards, mice, printers, monitors, routers, hubs and other external devices all snaking their wires around. You might find yourself wondering what makes one cable different from another. For example, take a look at the one going from your router to your high-speed modem. Take a look at the one going from the back of the printer to the computer. What makes them different?
First, there is that printer cable, which is probably USB 2.0. "USB" stands for "Universal Serial Bus," and the 2.0 version transmits data at 480 megabits per second (Mb/s) or 60 megabytes per second (MB/s). This is generally overkill for a printer, but USB ports are cheap and plentiful. USB 2.0 speeds are much more appropriate for something like an external DVD burner or external hard drive. However, the upcoming USB 3.0 design is estimated to be capable of 3.2 Gb/s of data transfer, or 400 MB/s.
Power and Connectors
It can also be used to transmit power, which is how a USB hub lights up. A small lamp can be adapted to draw its power from a USB port. Also, USB connectors can be shaped six different ways. Each cable has an "A" end and a "B" end. Then there is the standard connector, mini connector and micro connector. The latter two are popular for digital cameras and other portable devices. Ethernet just uses an RJ-45 jack, which is like a phone jack but larger.
Ethernet is a different beast. For one, it doesn't transmit power, only data. If you have a gigabit Ethernet connection, you're also going faster than USB 2.0. Gigabit Ethernet transfers data at up to 125 MB/s (one byte = eight bits), which makes it twice as fast as USB 2.0's theoretical maximum.
Also, an Ethernet cable can go about 100 yards without degrading the signal quality, while a USB cable usually can't go more than ten feet. That's one of the reasons why Ethernet is the standard for connecting stationary computing devices in a home or place of business. That, and Ethernet cables are much cheaper per foot than USB. Warehouse-style online stores can sell you 100 feet of "Cat 5e" Ethernet cable for less than $10. However, you can buy an "extender kit" that links two distant USB devices together with an Ethernet cable.
Most modern motherboards have a gigabit Ethernet controller built in. You can buy "10 gigabit Ethernet" cards, but they are generally only used in enterprise-level situations. Either way, gigabit Ethernet offers higher speeds than USB 2.0, until USB 3.0 is released some time in 2010. Typically, a computer will have one, maybe two Ethernet connections, but it will have 6-10 USB ports--some of them on the monitor, keyboard and front of the computer case.