In 1989, after experimenting with a number of innovative home-brewed networking technologies, the European Laboratory for Particle Physics (CERN) connected to the Internet. This research centre, located in Geneva, Switzerland and France, provided much of the impetus for the adoption of connectivity and distributed computing standards such as TCP/IP.
In the same year, Tim Berners-Lee proposed a hypertext system based on an earlier private project named "Enquire". Berners-Lee was a graduate in physics who had joined CERN in 1984 after a freelance career developing multitasking operating systems, typesetting software, and a variety of other applications. Over the next year, he wrote the first server and client software for what was to be known as the World Wide Web (WWW or W3). The system was first made available in December 1990, and refined over the period until 1993. At this time a standard for the Hypertext Markup Language (HTML) was established by the International Standards Organization (ISO). This is the standard language for hypertext creation on the Internet.
HTML is a subset of SGML which involves marking up ASCII text files with standard codes (example). This is typically done with a basic text editor, although dedicated authoring tools and scripts are becoming available to automate the task. The ISO standard, now at version 2.0, will certainly see a great deal of development over the next few years. Currently it allows for only the simplest of page layouts.
[2000 update: Let's just say that a lot has changed!]