Or you may have misspelled the third level domain name (see below)
A little background:
Domain names are read from right to left in sections divided with a dot or period.
All domain names start with a "root" or "first-level" domain on the right such as com, net, us, info, or org.
The next section (usually in the center) is called the "second-level" domain which designates the general company website you are looking for such as google, facebook, nytimes or hotmail.
Then there may (or may not be) a "third-level" domain. The most common third level domain is www, however, a third-level domain can be almost anything the system administrator wants to use.
If you have requested a third-level domain which does not exist on this server, (such as www) the server will send you to the first website in its list, which just happens to be this page.
This server contains several websites that do not use third level domain names. There are also many third-level domain names here, but none of them start with www.