What is the difference between http:// and http://www. ?

They are just different domain names. Many online site owners register both the domain names, for example http://cnn.com and http:// www.cnn.com. They point to the same site. However, there are also millions of sites where the non-www domain names are not registered.
If does not contain one of the dot-names such as ".com" or ".org", then the name is resolved within your LAN. So it is likely to point to a web site in the Intranet of your company.