When it comes to building a Web service, the Web Services Description Language (WSDL) is the most popular option for describing Web services. WSDL is the XML-based language for describing Web services.
It is possible to implement Web services in a variety of ways. For example, to send a message correctly, you need to know the following:
The IP address and other endpoint information
If using HTTP, the SOAPAction
The allowable set of messages (called operations) that the service supports
The schema (and possibly the SOAPAction) for the request message of each operation
The response schema to expect if there is a response
Any possible headers that may be expected in the request ...