It is a script which allows clients to make procedural calls over the net.
As it says in the name, the encoding is XML and because it is used on websites we can make the fair assessment that it uses the HTTP protocol.
If we break the name down we get: XML transmition via Remote Procedure Calls.
So you are still not sure what this is all about? Read on..
xmlrpc.com has a very simple yet rather effective diagram which illustrates what’s going on with it all:
As we can see above, xmlrpc is basically a simple way of transporting data using a common protocol used on the web.
This file is usually added to the root of a domain when using packages such as WordPress, Drupal or similar.
XMLRPC.php only accepts POST requests so using the CURL library or similar would be the best option when interacting with it. The PHP source file is quite long so I will not include it here, as it is 3375lines long. But you can easily find it online using your favorite search engine!