The query string is whatever you want it to be:
<a href="http://www.yoursite.com/main.php?page=bio">click me</a>
Then in file "main.php", you reference the value from the $GET array:
<?php
switch($_GET['page'])
{
case "bio":
include "pages/bio.html";
break;
case "links":
include "pages/links.html";
break;
default:
include "pages/default.html";
}
?>