The "Web," short for "World Wide Web" (which gives us the acronym www), is the name for one of the ways that the internet lets people browse documents connected by hypertext links.
The concept of the Web was perfected at CERN (Conseil Européen pour la Recherche Nucléaire, or the European Organization for Nuclear Research) in 1991 by a group of researchers which included Tim-Berners Lee, the creator of the hyperlink, who is today considered the father of the Web.
The principle of the Web is based on using hyperlinks to navigate between documents (called web pages) with a program called a browser. A web page is a simple text file written in a markup language (called HTML) that encodes the layout of the document, graphical elements, and links to other documents, all with the help of tags.
Besides the links which connect formatted documents to one another, the web uses the HTTP protocol to link documents hosted on distant computers (called web servers, as opposed to the client represented by the broswer). On the internet, documents are identified with a unique address, called a URL, which can be used to locate any resource on the internet, no matter which server may be hosting it.
A website (also called an internet site or a home page in the case of a personal site) is a group of HTML files that are stored on a hosting computer which is permanently connected to the internet (a web server).
A website is normally built around a central page, called a welcome page, which offers links to a group of other pages hosted on the same server, and sometimes "external" links, which lead to pages hosted by another server.
A URL looks something like this:
Let's take a closer look at this address:
http:// indicates that we want browse the web using the HTTP protocol, the default protocol for browsing the Web. There are other protocols for other uses of the internet.
www.commentcamarche.net corresponds to the address of the server that hosts the web pages. By convention, web servers have a name that begins with www., to make it clear that they are dedicated web servers and to make memorising the address easier. This second part of the address is called the domain name. A website can be hosted on several servers, each belonging to the same name: www.commentcamarche.net, www2.commentcamarche.net, intranet.commentcamarche.net, etc.
/www/www-intro.php3 indicates where the document is located on the machine. In this case, it is the file www-intro.php3 located in the directory www.