SOAP is a standard means of sending communications from one Web server to another, regardless of platform or Web server software being used. SOAP messages a standard designed to enable server communications via an XML-formatted message through the HTTP protocol. This communication is made up of a SOAP client and a SOAP server. The SOAP client sends a SOAP request to the SOAP server, which, in turn, sends back a SOAP response.
Getting Started with SOAP
Low-Level SOAP Details