Boy, do you ask the hard questions!
SOAP is an XML protocol. (What's an XML protocol?) An XML protocol is an XML messaging format that you can use to send information from one application to another application.
To put it simply, SOAP and its related technologies (WSDL, UDDI, etc.) form the foundation for a new type of middleware for application-to-application integration -- I usually refer to it as Intenet Middleware. SOAP lets you do application-to-application integration over the Web. Unlike previous middleware technologies (e.g., DCE, CORBA, DCOM, RMI, MOM, etc.) SOAP middleware is available on any platform, and it supports any programming language.
This was first published in September 2002