Normally, the practice or very old way of displaying some profile page is like this:
www.domain.com/profile.php?u=12345
where u=12345
There are lots of different ways to do this. One way is to use the RewriteRule techniques mentioned earlier to mask query string values.
One of the ways I really like is if you use the front controller pattern, you can also use urls like http://yoursite.com/index.php/path/to/your/page/here and parse the value of $_SERVER['REQUEST_URI'].
You can easily extract the /path/to/your/page/here bit with the following bit of code:
$route = substr($_SERVER['REQUEST_URI'], strlen($_SERVER['SCRIPT_NAME']));
From there, you can parse it however you please, but for pete's sake make sure you sanitise it ;)