If a web service is to be "callable" for clients from the outside world, the client need a description of the service interface. Without a description of the interface, how would the client know what data to send to the service?
You can think of a service interface like an interface in Java or C#. The only extra information needed is where the service is located (IP address), and the message format used by the service. Here is what a service description should contain:
How a web service interface description looks depends on the message format used by the web service. Currently, only SOAP web services has a standardized interface description - the Web Service Description Language (WSDL). WSDL will be described in its own trail later.