The 'login' in /index.php?login and the 'register' in /index.php?register are referring to variables that are submitted to the script via the URL. They are not different pages, but just variables that are sent to the script at index.php for processing. So you might see /index.php?login=true&username=pauleth. When the script receives this, it defines two variables with those values. Here is an example:
$login = $_GET['login']; // get the value stored in login in the URL
$user = $_GET['username']; // get the value stored in username in the URL
Now we have a variable called $login that contains the value 'true', and a variable called $user that contains the value 'pauleth'.
You may have entire portions of your script that are only executed if a variable holds a particular value. Hence, someone who is actually logged into a site may see something quite different than an unregistered guest, such as a welcome message like: "Welcome back, pauleth!" I hope that helps...