Before there was the Internet, there was the ARPAnet.
ARPAnet started as a Department of Defense (DoD) research project in survivable communications. In the sixties, there was a great deal of concern about the possibility of a nuclear war. Those responsible for planning for such contingencies knew that nuclear detonations would severely disrupt radio transmission and reception, which would make Command and Control (C2 or C-squared) of the military almost impossible.
The DoD agency that dealt with very futuristic research was (and is) the DoD Advanced Research Projects Agency, which has gone by the acronyms ARPA and DARPA variously throughout its existence. ARPA was interested not only in survivable communications but in computer networking. Killing two birds with one stone, they created a computer network between their various research contractors, which was the beginning of the ARPAnet, and the precursor to the Internet.
Originally linking four research centers - UCLA, Stanford Research Institute (now SRI, International), UCSB and the University of Utah - the ARPAnet came online in 1969. A huge number of technical problems had to be solved first, notably how to get computers from different manufacturers and operating systems to communicate. This led to the now-standard communication protocols, TCP/IP - transmission control protocol/internet protocol.
Information is still transmitted through networks today, as it was in those early days; it is broken up into small units called packets, which are sent out via whichever route is least busy, then reassembled at the destination. This routing of information can successfully traverse a network with significant damage, relies on no centralized control, and thus provides the 'survivability' model of communication that the DoD originally sought.
The ARPAnet very quickly spread to other computer research facilities that had contracts with the DoD, whether or not those contracts were with DARPA. In 1971, an email program was created, and in 1972, the @-symbol became a standard for designating where the individual user was 'at'. By 1973, it was calculated that seventy-five percent of the ARPAnet's network traffic was email.
The ARPAnet was startlingly successful at one of DARPA's secondary goals: increased communication between their various researchers. Mailing lists burgeoned overnight, on a wide variety of topics, not exclusively limited to computer science. One of the earliest, busiest mailing lists was SF-LOVERS, for devotees of science fiction.
Soon universities who did not have DoD research contracts realized the need to have a similar network and CSNET was born. Commercial service providers such as AOL, Prodigy and Compuserve soon followed, and the boom was on. When adoption of http (hypertext transmission protocols) created the World Wide Web, the Internet was fully formed.
When the original ARPAnet was finally shut down in 1990, the Internet was so widespread that its departure went virtually unnoticed.