SOAP is very similar to XML-RPC. It, too, works by marshaling procedure calls over HTTP as XML documents. Unfortunately, SOAP appears to be suffering from specification creep.
SOAP was originally created as a collaboration between UserLand, DevelopMentor and Microsoft. The initial public release was basically XML-RPC with namespaces and longer element names. Since then, however, SOAP has been turned over a W3C working group.
Unfortunately, the working group has been adding a laundry-list of strange features to SOAP. As of the current writing, SOAP supports XML Schemas, enumerations, strange hybrids of structs and arrays, and custom types. At the same time, several aspects of SOAP are implementation defined.
Basically, if you like XML-RPC, but wish the protocol had more features, check out SOAP. :-)