The Internet (Upper case I) is actually a network of networks that are connected to each other, using the TCP/IP protocol and other similar protocols.

Originally designed by the U.S. Department of Defense so that a communication signal could withstand nuclear war and serve military institutions worldwide, the Internet was first known as the ARPANet, the most robust communication technology. It is a system of linked computer networks, international in scope, that facilitates data transfer and communication services, such as remote login, file transfer (FTP), electronic mail (e-mail), newsgroups, and the World Wide Web. The Internet greatly extends the reach of each connected computer network (see: network effect, IP).

It is a three level hierarchy composed of backbone networks, mid-level networks, and stub networks. These include commercial (.com or .co), university (.ac or .edu) and other research networks (.org, .net) and military (.mil) networks and span many different physical networks around the world with various protocols, chiefly the Internet Protocol.

Until the advent of the World-Wide Web in 1990, the Internet was almost entirely unknown outside universities and corporate research departments and was accessed mostly via command line interfaces such as telnet and FTP. Since then it has grown to become an almost-ubiquitous aspect of modern information systems, becoming highly commercial and a widely accepted medium for all sort of customer relations such as advertising, brand building, and online sales and services. Its original spirit of cooperation and freedom have, to a great extent, survived this explosive transformation with the result that the vast majority of information available on the Internet is free of charge.

While the web (primarily in the form of HTML and HTTP) is the best known aspect of the Internet, there are many other protocols in use, supporting applications such as electronic mail, Usenet, chat, remote login, and file transfer.

Unlike online services, which are centrally controlled, the Internet is decentralized by design. Each Internet computer, called a host, is independent. Its operators can choose which Internet services to use and which local services to make available to the global Internet community. However, there are several bodies associated with the running of the Internet, including the Internet Architecture Board, the Internet Assigned Numbers Authority, the Internet Engineering and Planning Group, Internet Engineering Steering Group, and the Internet Society.

There are a variety of ways to access the Internet. Most online services, such as America Online, offer access to some Internet services. It is also possible to gain access through a commercial Internet Service Provider (ISP).

internet (lowercase "i") usually refers to a group of local area networks (LANs) that have been connected by means of a common communications protocol. Many internets exist besides the Internet, including many TCP/IP-based networks that are not linked to the Internet (the Defense Data Network is a case in point).