"AS" is mainly for "JOIN" clauses ( LEFT JOIN, INNER JOIN, etc.)
If you join two tables that have the same field name you can "AS".
Both the "user" and "usergroup" tables have the field "usergroupid".
$getusers = mysql_query("
SELECT u.usergroupid AS u_groupid, ug.usergroupid AS ug_groupid
FROM user AS u
LEFT JOIN usergroup AS ug
ON u.usergroupid = ug.usergroupid ");
$user = mysql_fetch_array($getusers);
echo "Usergroup ID: ".$user['ug_groupid']." <br />
User ID: ".$user[u_groupid']."";
Sorry for the bad explanation but hopefully you'll get it.