XML Web service
XML Web services are the fundamental building blocks in the move to distributed computing on the Internet. Open standards and the focus on communication and collaboration among people and applications have created an environment where XML Web services are becoming the platform for application integration. Applications are constructed using multiple XML Web services from various sources that work together regardless of where they reside or how they were implemented.
There are probably as many definitions of XML Web Service as there are companies building them, but almost all definitions have these things in common:
XML Web Services expose useful functionality to Web users through a standard Web protocol. In most cases, the protocol used is SOAP.
XML Web services provide a way to describe their interfaces in enough detail to allow a user to build a client application to talk to them. This description is usually provided in an XML document called a Web Services Description Language (WSDL) document.
XML Web services are registered so that potential users can find them easily. This is done with Universal Discovery Description and Integration (UDDI).