The referrer value indicates the page that was being viewed when the user clicked on a link to request this document.
The referrer data is often used in log analysis to trace a click stream that the user traversed your web site with. This property allows you to do similar things at the client end and build all kinds of history trees and back button simulations.
If you are prepared to do a little work to dismantle the referring link, you can detect when someone has a link to one of your internal pages. What you do then is up to you but you could redirect to an error page or simply bump the user to your front page. This would mean that whatever they did to bookmark one of your pages, they would always end up at your front door instead.
This does not work in MSIE version 3.
In Netscape , the referrer only contains a value if the document is different. If a document refers to itself in an anchor, clicking on that anchor effectively reloads the document. The referrer will be an empty string. It continues to to be suspect at Netscape 6.0 as well.
Some log analysis studies suggest that the referrer value is empty in MSIE when documents are loaded into frames.
Also as a result of studying log results, book-marked links or link values typed into the location box or drag/dropped into the browser will also not yield a meaningful referrer value.