ARPANET is an acronym for Advanced Research Projects Administration Network, the computer network system that gave birth to the Internet.
ARPANET began in 1969 as a U.S. Department of Defense experiment in packet-switched networking. There is some debate over the exact reason the Department of defense created ARPANET, whether it was purely a defensive measure, or if it was meant to further scientific research, but one can assume that both objectives played a role in its development.
At the height of the cold war, the ability to communicate between military bases using a protected system was deemed vital to national security. At the same time, powerful research computers were available to only a handful of universities in the United States, making it impossible for most scientist to benefit of their abilities, due to their geographical location. ARPANET was a means to connect scientists from across the country and allow them access to these advanced research machines, even if they could not get there in person.
Much like the modern Internet, ARPANET was a system of computers connected together using the same protocols. Originally, ARPANET was only comprised of a small number number of computers from the scientific and military communities. As more systems joined the network, new protocols were created to allow communication between different computer systems. Many of these protocols are still in use today, including TCP/IP, which was created for ARPANET in 1982 and it still the backbone of the modern Internet.
By the early 1980s, ARPANET had expanded to hundreds of computers across the globe, and the military made the decision to split off and create their own, more secure network, known as MILNET. At the same time, many universities began creating their own internal computer networks, known as LANs, or local area networks. In the mid-1980s, one of these LANs created an alternative to ARPANET, known as NSFnet, the National Science Foundation Network. NSFnet was significantly faster than its predecessor and quickly replaced the older network. In 1990, ARPANET was shut down.
Also See: Birth of the Net