The Web or World Wide Web (WWW), is the part of the Internet made up of web pages with links (hyper links) to other pages on the Internet and written in a language called HTML which you need a web browser to read properly.
If you go to just about any website you will be taken to a page known as the homepage and from this page will be links to other pages. This simple concept of pages linking to other pages is the World Wide Web.
The web as we know it today was created by British engineer and computer scientist Sir Tim Berners Lee.
It is common to add the www prefix to domain names, but becoming less so. More often than not, you can leave out the www portion of a domain name and still get to the same website. The www part of the domain name is called a sub domain.