Can Someone Explain this error message to me?

Of all the things I have tackled this far with PHP, user authentication has eluded me the most. I understand the basic concepts of security and am even able to create a sinple logon scrip that puttls data from a MySQL table. but when it comes to session, I guess I just can't grasp the whole sessions concept. I went to a website to try and get some help through some sample code. I have the script installed and have defined the variables in the .inc filed. I am getting the following errors when I try to run it and I don't understand what it is trying to tell me. I have also pasted the section of the script that I believe this is referencing. If someone could have a look and try to break this down into non-techno-ese for me I would appreciate it. Thanks, Jess Warning: Cannot add header information - headers already sent by (output started at /var/www/testbed/login.inc:8) in /var/www/testbed/login/login.inc on line 14 Warning: Cannot add header information - headers already sent by (output started at /var/www/testbed/login.inc:8) in /var/www/testbed/login/login.inc on line 15 $userInfo = mysql_query($sql); if($user = mysql_fetch_object($userInfo)) { setcookie(userID, "" . $Username); header("Location: $afterLoggedIn?user=" . $user->$useridField); } else { $error = "$errorMessage"; } } if(($error || !$userID == $user || $userID=="" ) && !$action == "register" ) {