Ethernet (pronounced eether net) is the most common networking system for local area networks, called lans, which means the computers that are connected together are close enough to be connected with cables (as opposed to a wide area network, a wan). iMacs have an Ethernet port; it looks like a large phone jack. You can use Ethernet even in your home or small office to connect several computers so you can send files back and forth.
Your printer might have an Ethernet port, in which case you can send data to the printer much faster than through the usb port. If you have a cable connection to the Internet or dsl (digital subscriber line), you'll use the Ethernet port to connect to the special cable or dsl modem, instead of the phone port described below.