Internet
The internet is the global system of interconnected computer networks that exchange data using the TCP/IP family of protocols, and it is best understood as a network of networks: thousands of independently operated networks run by universities, companies, carriers and governments agree to pass one another's packets, so that any connected device can in principle reach any other. No single organization owns it, and what holds it together is a shared set of open standards rather than a central authority.
The internet began as the ARPANET, a research network funded by the US Department of Defense's Advanced Research Projects Agency, whose first host-to-host message was sent between UCLA and the Stanford Research Institute in 1969. That first transmission was a login attempt that crashed after the first two letters, "lo". The ARPANET used packet switching, with BBN-built Interface Message Processors acting as the routing nodes, and it ran on an early protocol called NCP. By the early 1970s Vint Cerf and Bob Kahn were designing a way to join dissimilar networks, publishing their landmark paper on the idea in 1974, and the resulting Transmission Control Protocol was later split so that IP handled addressing and routing while TCP handled reliability. The specifications appeared as RFC 791 (IP) and RFC 793 (TCP) in 1981.
The decisive moment was the flag day of 1 January 1983, when the ARPANET switched from NCP to TCP/IP, and for this reason that date is commonly given as the birth of the modern internet. Every host had to convert by that day or lose connectivity. The same period produced the Domain Name System (designed by Paul Mockapetris in 1983), which replaced numeric addresses with memorable names, and the NSFNET backbone of the mid-1980s, which connected universities across the United States and led to commercial networks and, in the early 1990s, public access. The Internet Engineering Task Force develops the standards through open documents called RFCs, ICANN (created in 1998) coordinates names and numbers, and routers exchange reachability information between networks using the Border Gateway Protocol.
The internet is the infrastructure, and the World Wide Web is only one service that runs on it, which is the most widespread misconception about the subject. Tim Berners-Lee proposed the web at CERN in 1989 and built the first browser, server and pages in 1990 and 1991, using HTTP and HTML; it reached a mass audience with the Mosaic browser in 1993. Email, file transfer, SSH remote logins, DNS lookups, online games, video calls and software updates all use the internet without using the web. Another confusion is between the internet and a particular access technology: Wi-Fi, a phone plan or a fiber connection is only the way of attaching a device to the internet, not the internet itself. Connecting to the internet means being reachable through the routing system, usually through an internet service provider.
Because the internet was designed to be open, it moves packets on a best-effort basis, with intelligence at the edges rather than in the middle, and this design explains both its growth and its weaknesses. Anyone can build a new application without asking permission, which allowed the web, streaming and cloud computing to appear on the same base. The cost is that delivery is not guaranteed, that addresses are scarce (the 32-bit IPv4 space is exhausted, and IPv6 provides a vastly larger one), and that security such as encryption through TLS had to be added afterward on top. Today the internet carries the data of search, social platforms, scientific collaboration and AI services, and the training and serving of machine learning models depend on the same packets that carry any other traffic.
Read the internet as an agreement rather than a machine: a loose federation of independent networks that all speak TCP/IP, born on the ARPANET in 1969 and unified by the switch-over of 1 January 1983, upon which the web, email and every other online service are only the tenants.