What is the World Wide Web?
To understand what the World Wide Web really is, you must think of the World Wide Web as an application (a computer program) that uses the Internet. That program is called a web browser. A web browser is a piece of software that sends requests to web servers and makes pretty web pages out of the data the servers send back. To someone using a web browser, it appears that you are able to browse the files on the remote server by using a mouse to click on things called hyperlinks.
Web browsers use the Internet (a network of computers) to reqeust and receive information from a web server. Your web browser then presents the information to you in a pretty and user-friendly fomat. The WWW exists as a hodgepodge of files on computers scattered all over the world. The World Wide Web is NOT a network.
What you think of as "Yahoo" or "Google" is actually a bunch of files and scripts running on a remove computer called a web server. The web server delivers data from Yahoo's headquarters to your computer at the office or home over the Internet. Without the Internet, the "Web" or "WWW" wouldn't exist.
There are a number of technologies and protocols that go into making the whole WWW experience work.