Also known as PHPXMLRPC, the XML-RPC protocol is used in WordPress, Drupal, WikiTeX, TikiWiki, OpenAutonomy, Xaraya, Ampache, PostNuke, and many other open source projects. The XML-RPC protocol was originally created in PHP for ease of use and flexibility.
WordPress documentation about XML-RPC is rather poor, but we will try to tell you how it works in more detail.