XML-RPC is an alternate web services technology, a simple technique for invoking an application's method or procedure by sending XML documents in HTTP packets. Sound kind of like SOAP? Well, XML-RPC is the predecessor of SOAP. Apparently Microsoft thought XML-RPC was too simple, added more functionality, and gave the more complex technology the perhaps misleading name/description: Simple Object Access Protocol (SOAP).
XML-RPC has gone out of favor. People seem to prefer either SOAP or REST instead.
For more info:
- XML-RPC Specification
- Apache XML-RPC
- XML-RPC How To by Eric Kidd
- XML-RPC vs. SOAP by Kate Rhodes
- XML-RPC for Newbies by Dave Winer