A postback is the term to refer to data that has been POSted back by the client to the server. For example when you submit data on an HTML form via your web browser (which is the client here), when you submit that form, a postback to the server occurs. Effectively, the web browser, with its HTTP request for the specified file, will also have the postback data. If you have a PHP application, by using the $_POST and $_GET superglobal variables (or $_REQUEST to refer to $_POST, $_GET and $_COOKIE in one), you can retrieve the postback data within your PHP applications.
A primary example of a postback is a login or registration form – the postback data will consist of the username and password, and any other information fields you require on either forms.