From the FAQ document:
WSDL is an XML format for describing network services as a set of endpoints operating on messages containing either document-oriented or procedure-oriented information. The operations and messages are described abstractly and then bound to a concrete network protocol and message format to define an endpoint. Related concrete endpoints are combined into abstract endpoints (services). WSDL is extensible in order to allow description of endpoints and their messages regardless of what message formats or network protocols are used to communicate.
Why should I use a tool like WSDL Explorer? If you plan to use externally developed Web services in a project, you will need a means of analyzing candidate services. You can use a UDDI browser tool to locate services and references to their WSDL files in a UDDI directory (or directories). However, once you locate a service, the task of analyzing its functionality requires more effort than simply reading the service's WSDL description. Ideally, you would like to be able to see available bindings, list and invoke operations on the service, and look at sample message flow. You would also like do this without any development investment: You don't want to import the service into your favorite development tool, create lots of artifacts, and spend time writing sample clients, only to find out that the service doesn't really meet your needs. WSDL Explorer allows you to see available bindings, list and invoke operations on services, and look at sample message flow for multiple services at once without having to import the services into your development tools.