The Net
The Internet, often called simply "the Net," is a world wide system of computer networks and, in a larger sense, the people using it. The development of the basic concept was mainly an initiative of the United States Government in 1969. The network should be an assurance that data would not get lost during a (nuclear) war by remaining in function even if large parts of it were to be destroyed.

World Wide Web (WWW)
Although being a subset of the Internet, the World Wide Web is probably best known to the larger public. Actually, the World Wide Web refers to all the resources and users on the Internet that are using the Hypertext Transport Protocol (HTTP). Apart from HTTP there are also other transport protocols for the Internet, of which the File Transport Protocol (FTP) is probably best known.
The name "World Wide Web" was introduced in 1990 by Tim Berners-Lee, the man who helped found the World Wide Web Consortium (W3C). This vendor-neutral consortium seeks to promote standards for the evolution of the WWW by producing specifications and reference software.

HTML:
The "universal language" for computers to communicate over the WWW is the Hypertext Markup Language (HTML). HTML basically is an ASCII text file, and thus can be produced in simple notepad software. To be able to view the HTML files special software is needed, often referred to a the "browser".
In brief, a browser is your interface to the World Wide Web; it interprets hypertext and lets you view sites and navigate from one WWW-page to another or from one Internet node to another. Among the most well known browsers are NCSA Mosaic, Netscape Navigator and Communicator, and Microsoft Internet Explorer.