Hi All you php people

I have a question if anyone could help with the answer there a brownnie point

When a mod-rewrite is done where are the words picked up from ie
http://www.thesitebox.com/shopWebSit...Page.php?cat=4
should read something like thesitebox.com/power_tools/4 or what ever

Is the code replaced by the page title or h1 header or something like that or does it work in a complete diffrent way.

Any info will be of a help just so I can understand a little of how it works.

Thanks

Ed