The main components of a Web Service are:

• Extensible Markup Language (XML). XML is a flexible text format that allows the exchange of data on the Internet, intranets, and elsewhere. It is a standard of the World Wide Web Consortium (W3C).
• Simple Object Access Protocol (SOAP). SOAP is a communications protocol designed for exchanging information in a distributed environment. It provides a messaging framework that is independent of implementation specifics, and it enables a program on one kind of operating system (such as Windows 2000) to communicate with a program on another kind of operating system (such as Linux). SOAP uses the Hypertext Transfer Protocol (HTTP) and XML to exchange information.
• Web Services Description Language (WSDL). WSDL, expressed in XML, describes how to access a Web Service and the operations that it will perform. WSDL, which was co-developed by Microsoft and IBM, describes the protocols and formats used by the Web Service.
• Universal Description, Discovery, and Integration (UDDI). UDDI is the component that enables businesses to find each other on the Web and make their systems interoperable for e-commerce. A business can list itself by name, product, location, or by the Web Service it offers. Information is entered on a registry server and then shared by servers in other businesses. Services are defined through a UDDI document called a Type Model or tModel. The tModel often contains a WSDL file that describes a SOAP interface to an XML Web Service.

You can use XML, SOAP, WSDL, and UDDI together to integrate Web-based applications. Use XML to tag the data, SOAP to transfer the data, WSDL to describe the available services, and UDDI to list the available services.