A software component or system designed to support interoperable machine- or application- oriented interaction over a network. A Web service has an interface described in a machine-processable format (specifically WSDL). Other systems interact with the Web service in a manner prescribed by its description using SOAP messages, typically conveyed using HTTP with an XML serialization in conjunction with other Web-related standards.
NIST SP 800-95
(Open Grid Services Architecture Glossary of Terms (25 January 2005))