When a client such as an application or a Web page uses a Web Service, this is know as consuming it. Web Services Description Language (WSDL) describes the Web Service to the client so it knows what classes and methods are available along with the location on the Internet. WSDL is another XML-based language.
WSDL is especially important with technologies from Microsoft. A WSDL file must exist for a Microsoft Web Service to consume any methods. With some of the Java technologies, it is possible to sometimes use a Web Service by just knowing the URL of the Web Services. Chapter 4 explores the syntax and many uses of WSDL.