DARPA (Defense Advanced Research Projects Agency) is the independent research branch of the U.S. Department of Defense that funded a project that in time was to lead to the creation of the Internet. Originally called ARPA (the "D" was added to its name later), DARPA came into being in 1958 as a reaction to the success of Sputnik, Russia's first manned satellite. DARPA's explicit mission was (and still is) to think independently of the rest of the military and to respond quickly and innovatively to national defense challenges.
In the late 1960s, ARPA provided funds and oversight for a project aimed at interconnecting computers at four university research sites. By 1972, this initial network, now called the ARPANET, had grown to 37 computers. Because ARPA's name was changed to Defense Advanced Research Projects Agency (DARPA) in 1971, some people refer to ARPANET as DARPANET. (DARPA was changed back to ARPA in 1993 and back to DARPA again in 1996.)