Web services are simple, self contained applications which perform functions, from simple requests to complicated business processes. The "web services" model uses WSDL, UDDI and SOAP/XMLP. A WSDL description is retrieved from the UDDI directory. WSDL descriptions allow the software systems of one business to extend to use those of the other directly. The services are invoked over the World Wide Web using the SOAP/XMLP protocol. Each of the components is XML based.
From : www.agimo.gov.au/publications/2005/04/agtifv2/glossary