XML-RPC is a remote procedure call protocol encoded in XML. It is a very simple protocol, defining only a handful of data types and commands, and the entire description can be printed on two pages of paper. This is in stark contrast to most RPC systems, where the standards documents often run into the thousands of pages and require considerable software support in order to be used.
It was first created by Userland Software in 1995, and later picked up by Microsoft. However Microsoft considered it too simple, and started adding functionality. After several rounds of this the standard was no longer so simple, and has re-emerged as SOAP.