The Internet is a worldwide collection of networks that connects millions of  businesses, government agencies, educational institution, and individuals. People use the internet to communicate with and meet other people, to access news and information, to shop for goods and services, to bank and invest, to take classes, and to access sources of entertainment and leisure. The Web, short for World Wide Web, is a global library of documents containing information that is available to anyone connected to the Internet.

domains are set up in a hierarchical structure, each segment (between the dots) represents one level of the hierarchy. The right-most is the highest (most significant) segment. Each segment directly to the left of a given segment is said to 'belong to' or to 'be a part of' that segment. For example, with www.networkactiv.com, the most significant domain segment is 'com' (at the right), the next most significant is 'networkactiv' (in the middle), and the least significant is 'www' (at the left). I know the 'www' thing being the least significant may seem counter-intuitive, and this will be explained in a moment. This all means that in the example of www.networkactiv.com, 'networkactiv' belongs to (is a part of) 'com' and 'www' belongs to (is a part of) 'networkactiv'. Whoever controls a given segment also controls the segments to its left. If you own 'something.com', you also own 'www.something.com' and 'anythingelse.something.com'. This is because the DNS server (the computer running the DNS server software) responsible for a given 'something.com' is what handles providing the information of what IP address anything to the left of 'something.com' has. I know I hadn't yet mentioned it, but each composition of a domain name, that is, 'something.something.something' (however many segments) represents one (or sometimes more) IP addresses. This means that 'www.google.com' and 'google.com' could actually represent different IP addresses. What does this mean in practice?... It means that each could have a different Web site because each could represent a completely different computer located in a completely different place. Even if they have the same IP address, it's still possible for them to have different Web sites, but likely that they will be on the same computer, but that will be explained later in this tutorial. The DNS system was designed for more than just Web pages... The 'www' at the left of most domain compositions is supposed to mean, "The IP address of whatever.com that represents the computer hosting the Web site for whatever.com.". Basically, by putting the 'www.' at the left, you are saying that you want the Web site (hence, WWW (World-Wide-Web)) for that domain, and not something else.

ref:  http://www.networkactiv.com/WebSiteAndWebHostingBasicsTutorial.html