A very basic example of how it works :
<?php
$page = $_GET['page'];
if ( $page == 'one' ) {
echo 'This is page one!';
} elseif ( $page == 'two' ) {
echo 'This is page to';
}
// http://yoursite.com/index.php?page=one
// Outputs 'This is page one!'
Learn little more about GET request right here : $_GET
solved Multiple HTML Pages in one file