/SECTION in a URL

[flynch01]

I'm kindof confused as to how you find out what the /thisbit of a link is. For example.

youtube.com/profilename
xfire.com/profile/profilename

Because if you try to access it as a directory or file it just says not found. So there must be a way of catching that part of the url. Theres no ? before it so surely its not being sent through GET. And POST can't be either because you can access it through your url bar. Is it literally just stored in the root post variable or something?

[_Aerospace_Eng_]

Its likely using mod_rewrite. Look it up. It will explain a lot. Another term to search for is clean URLs.