ARPANET stands for Advanced Research Projects Agency Network, and was created by DARPA (Defensive Advanced Reasearch Projects Agency), a branch of the Department of Defense. It was developed by computer scientist J.C.R Licklider in August 1962. It is the predecessor of the modern day internet, and was used for basic communication between computer users.
In the mid-1960s, four IMPs (Interface Message Processors - terminals for communications) were set up for testing. They were located at UCLA, Stanford's Research Institute, UC Santa Barbara, and the University of Utah. The first message was sent by a UCLA computer science student to Stanford's Research Institute on October 29, 1969.
The first message was "login"; however, only the "l" and the "o" were sent before the system crashed. On November 21, 1969, the first permanent message was successfully sent from UCLA to Stanford's Research Institute. Although it has been forgotten what the message actually said, it is theorized that the message was "QWERTYUIOP", the top row of a keyboard.