SOAP

http://www.w3schools.com/soap/default.asp

SOAP provides a way to communicate between applications running on different operating systems, with different technologies and programming languages.

A SOAP message is an ordinary XML document containing the following elements:

An Envelope element that identifies the XML document as a SOAP message
A Header element that contains header information
A Body element that contains call and response information
A Fault element containing errors and status information

The optional SOAP Header element contains application-specific information (like authentication, payment, etc) about the SOAP message.

If the Header element is present, it must be the first child element of the Envelope element.The attributes defined in the SOAP Header defines how a recipient should process the SOAP message.

If a Fault element is present, it must appear as a child element of the Body element. A Fault element can only appear once in a SOAP message.