SOAP is the protocol of Web services. It forms the basis of the way in which messages are sent. The SOAP protocol defines
An enveloping mechanism consisting of elements (in the SOAP namespace) within which any XML can be sent.
An extensibility hook for <Headers>, including targeting of those headers at specific actors, or intermediaries, as well as force errors if those headers are understood with the mustUnderstand attribute.
An RPC mechanism, as well as a data-encoding model.