The World Wide Web (WWW) is combination of all resources and users on the Internet that are using the Hypertext Transfer Protocol (HTTP).
A broader definition comes from the World Wide Web Consortium (W3C):
"The World Wide Web is the universe of network-accessible information, an embodiment of human knowledge."
The Web, as it's commonly known, is often confused with the internet. Although the two are intricately connected, they are different things. The internet is, as its name implies, a network -- a vast, global network that incorporates a multitude of lesser networks. As such, the internet consists of supporting infrastructure and other technologies. In contrast, the Web is a communications model that, through HTTP, enables the exchange of information over the internet.
Tim Berners-Lee is the inventor of the Web and the director of the W3C, the organization that oversees its development. Berners-Lee developed hypertext, the method of instant cross-referencing that supports communications on the Web, making it easy to link content on one web page to content located elsewhere. The introduction of hypertext revolutionized the way people used the internet.
In 1989, Berners-Lee began work on the first World Wide Web server at CERN. He called the server "httpd” and dubbed the first client "WWW.” Originally, WWW was just a WYSIWYG hypertext browser/editor that ran in the NeXTStep environment.
The World Wide Web has been widely available since 1991.