Web Services Description Language (WSDL)
WSDL is an XML format for describing network services as a set of endpoints operating on messages containing either document-oriented or procedure-oriented information. The operations and messages are described abstractly, and then bound to a concrete network protocol and message format to define an endpoint. Related concrete endpoints are combined into abstract endpoints (services).
For more information: {{{ specification
[wiki:/XmlSchema XML Schema]
[http://webservices.xml.com/lpt/a/ws/2004/05/19/wsdl2.html What's New in WSDL 2.0] }}}