From www.xmlrpc.com:

What is XML-RPC? It's a spec and a set of implementations that allow software running on disparate operating systems, running in different environments to make procedure calls over the Internet.

It's remote procedure calling using HTTP as the transport and XML as the encoding. XML-RPC is designed to be as simple as possible, while allowing complex data structures to be transmitted, processed

Whith XML-RPC you can do a lot of usefull :P stuff like this: