The Internet is made up of many thousands of smaller national and regional networks that link together to form a global network.
All of these networks use TCP/IP (Transmission Control Protocol/Internet Protocol). This is a set of rules which allow the computers that are connected to a network to interact with each other whether they are PCs, Macintoshes or UNIX machines.
Any network can join the Internet as long as it uses the Internet Protocol. No one controls the Internet, it works because these smaller networks co-operate with each other.
The Internet used to be primarily an education and research network but it is now becoming much more commercially oriented.