Web Services Description Language
WSDL (1.1) is an XML document and core part of the web services that describes the six elements:
1. Definitions - the root element that defines the name of web services and all the namespaces used in the WSDL document.
2. Types - container for data type definitions (such as XSD ) and describes the data types exchanged between the client and the server. The default typing system used is W3C XML Schema.
3. Message - element defines the name of message and the data type of each name (string, float etc.)
4. PortType - element defines multiple operations and can combine multiple elements into one-way or two-way operation (input, output).
5. Binding - element defines exact specification of the implementation of the services ( SOAP )
6. Services - defines the address ( URL ) for invoking the defined (SOAP) service.

In WSDL 2.0, the message element has been removed and revised as shown below in the WSDL 2.0 document layout.
WSDL 2.0 Document Layout 
<wsdl:description xmlns:wsdl="http://www.w3.org/ns/wsdl">
  <wsdl:types/>
  <wsdl:interface/>
  <wsdl:binding/>
  <wsdl:service/>
</wsdl:description>

Reference
1. W3C Web Services Description Language - WSDL 1.0 (15 March 2001)
2. W3C Web Services Description Language - WSDL 2.0 (26 June 2007)
3. Ethan Cerami, Web Services Essentials, Distributed Applications with XML-RPC, SOAP, UDDI & WSDL, February 2002, O'Reilly Media, Inc.

Last Revised on: November 22, 2013