Not so long ago, computers were all made by different companies, worked in different ways, and couldn't communicate with one another. Often, they didn't even have the same sorts of plugs and sockets on their cases! During the 1980s and 1990s, everything became much more standardized and it's now possible to connect virtually any machine to any other and get them exchanging data without too much effort.
That's largely because most networks now use the same system, called Ethernet. It was developed in May 1973 by US computer engineer Dr Robert ("Bob") Metcalfe (1946–), who went on to found 3Com and later became a well-known computer-industry pundit (perhaps, somewhat unfairly, best known for predicting a spectacular collapse of the Internet in 1995 that never actually occurred).
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As Metcalfe originally designed it, Ethernet was based on three very simple ideas. First, computers would connect through the "ether" (a semi-serious, semi-scientific name for the void of emptiness that separates them) using standard coaxial cable (wires like the ones used in a television antenna connection, made up of concentric metal layers). In Ethernet-speak, the physical connection between the nodes (computers and other devices) on the network is also known as the medium. Things have moved on quite a bit since the early 1970s and the medium is now just as often a wireless radio link (you've probably heard of Wi-Fi, which is the wireless version of Ethernet). Second, all the computers and devices on a network would stay silent except for when they were sending or receiving messages. Finally, when they wanted to communicate, they'd do so by breaking up messages into small packets of data and sending them around the network by a highly efficient method known as packet switching (discussed in much more detail in our article on the Internet).