From Wikipedia:XML-RPC : XML-RPC is a remote procedure call protocol which uses XML to encode its calls and HTTP as a transport mechanism. 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.
XML-RPC is a remote procedure call mechanism for use in building XMLWebServices. It is lighter weight than SOAP, and hence more suitable for some low-end clients such as PDAs and Java phones.