WWW, or the World Wide Web, also W3 or "the Web" is a collection of interconnected hypertext pages (often called webpages or websites) connected to and using various internet protocols.  The WWW is accessed using a web browser.

The framework (HTML) for the WWW was developed in 1990 by Tim Berners-Lee and Belgian computer scientist Robert Cailliau,  In which they proposed  "to link and access information of various kinds as a web of nodes in which the user can browse at will".

On 12 March 1989, the now Sir Tim Berners-Lee wrote his influential paper “Information Management: A Proposal”[CERN link to original paper converted to HTML] 

A screen shot of the first WWW page accessible via the internet. image: http://info.cern.ch/hypertext/WWW/TheProject.html which is also the first URL recently restored CERN.ch.

The very first web page went live on  6th August 1991.  Later on the 23rd of August , 1991  people outside of CERN were invited to join the web. This is sometimes referred to as internaut day, presumably a reference to the term _naut, like astronaut.   In April 1993, CERN announced the decision to make the underlying code free to use. 

Further reading