The mother of all networks. First incarnated beginning in 1969 as the ARPANET, a U.S. Department of Defense research testbed. Though it has been widely believed that the goal was to develop a network architecture for military command-and-control that could survive disruptions up to and including nuclear war, this is a myth; in fact, ARPANET was conceived from the start as a way to get most economical use out of then-scarce large-computer resources.
As originally imagined, ARPANET's major use would have been to support what is now called remote login and more sophisticated forms of distributed computing, but the infant technology of electronic mail quickly grew to dominate actual usage. Universities, research labs and defense contractors early discovered the Internet's potential as a medium of communication between humans and linked up in steadily increasing numbers, connecting together a quirky mix of academics, techies, hippies, SF fans, hackers, and anarchists. The roots of this lexicon lie in those early years.
Over the next quarter-century the Internet evolved in many ways. The typical machine/OS combination moved from DEC PDP-10s and PDP-20s, running TOPS-10 and TOPS-20, to PDP-11s and VAXes and Suns running Unix, and in the 1990s to Unix on Intel microcomputers. The Internet's protocols grew more capable, most notably in the move from NCP/IP to TCP/IP in 1982 and the implementation of Domain Name Service in 1983. It was around this time that people began referring to the collection of interconnected networks with ARPANET at its core as "the Internet".
The ARPANET had a fairly strict set of participation guidelines - connected institutions had to be involved with a DOD-related research project. By the mid-80s, many of the organizations clamoring to join didn't fit this profile. In 1986, the National Science Foundation built NSFnet to open up access to its five regional supercomputing centers; NSFnet became the backbone of the Internet, replacing the original ARPANET pipes (which were formally shut down in 1990). Between 1990 and late 1994 the pieces of NSFnet were sold to major telecommunications companies until the Internet backbone had gone completely commercial.
That year, 1994, was also the year the mainstream culture discovered the Internet. Once again, the killer app was not the anticipated one - rather, what caught the public imagination was the hypertext and multimedia features of the World Wide Web. Subsequently the Internet has seen off its only serious challenger (the OSI protocol stack favored by European telecom monopolies) and is in the process of absorbing into itself many of the proprietary networks built during the second wave of wide-area networking after 1980. By 1996 it had become a commonplace even in mainstream media to predict that a globally-extended Internet will become the key unifying communications technology of the next century. See also the network and Internet address.