The Web-service interface acts as a liaison between the Web and the actual application logic that implements the service. Most Web services communicate by passing XML messages. The Web-service interface runs in a Web server, but the application logic that implements the service can run anywhere. The application logic can be a new application written specifically to support the Web service, or it might be an encapsulated legacy application. These technologies are the precursors to standards that will enable Web services.
Soap is an extensible XML messaging protocol that forms the foundation for most Web services.
There are more than 50 Soap implementations currently available for a wide assortment of programming languages. Many implementations are open-source. WSDL is an XML language for describing a Web-service interface. WSDL tools can either generate Soap interfaces from WSDL descriptions or generate WSDL descriptions from application code. UDDI is a Web-based business registry that can be used to describe and discover Web services.