XML-RPC & SOAP
XML-RPC is a simple protocol for executing RPC with an XML message. The service request uses XML to encode and transmitted by HTTP POST, and the XML response is embedded in the HTTP response body. SOAP is an XML-based protocol exchange information between computers, mainly focusing on HTTP transmission RPC. It uses XML namespace and XML schema (XML Schema). XML-RPC is the easiest way to start web services, more simple easier than SOAP, but different from SOAP is that XML-RPC has no corresponding service description syntax, which hinders automatic calls for XML-RPC services.
-Than Cerami Webservices Essentials