The client application, mind you, can be any application that can issue an HTTP request to the Web service. That is, the client can be what we typically think of as a client - say a stand-alone Windows desktop application - or it can be what we more traditionally think of as a server - such as an ASP.NET Web page on a Web server.
Recall from our discussions in Part 2 that Web services communicate via messages formatted in the SOAP standard, where the messages are transmitted over the Internet using (typically) the HTTP protocol. A Web service call begins with the client sending a request for a particular method to the Web service. The Web service then processes this request and returns the result of the method wrapped up in a SOAP message. (See this diagram for a graphical illustration of a Web service interaction.) Also, in Part 2 we briefly discussed what WSDL was. Namely, is it the means by which a Web service is formally defined. It is the WSDL document that specifically spells out how the messages that will be passed to and from the Web service must be serialized. That is, how the method to be called is specified, and how the input and output parameters are represented in XML.
