ARPANET, or the Advanced Research Projects Agency Network, was the first operational packet-switching network and a precursor to the modern internet. Developed in the late 1960s by the U.S. Department of Defense, it facilitated communication between various universities and research institutions. ARPANET employed a decentralized architecture, allowing multiple computers to communicate simultaneously, which laid the groundwork for future networking technologies. It was officially decommissioned in 1990, but its legacy continues to influence the Internet today.