How to mask a URL

I am just a lonely Linux System Administrator and definitely starting to learn as a drupal webmaster. But for some reason I was given on this kind of task. I've been searching on Google for the past few days to address this problem on masking a url. Usually, I get suggestions to do it like mod_rewrite, frameset and iframes. I already tested it all but the url just freezes.

But what we want to achieved is to have a mask url that changed its link. As an example, for a mask url like http://www.example.com when you click the menu or any other links its just freezes on http://www.example.com. But what we want is when you click on "Contact Us", the url now then becomes http://www.example.com/contact_us.php.

Is this possible? I really and badly need your help. Thanks a million in advance.