(1) A Web service is a software system identified by a URI [RFC 2396],
whose public interfaces and bindings are defined and described using XML.
Its definition can be discovered by other software systems. These systems
may then interact with the Web service in a manner prescribed by its
definition, using XML based messages conveyed by Internet protocols.
-- Web Services Architecture Requirements (W3C Working Group Note 11 February 2004) (http://www.w3.org/TR/wsa-reqs/)
(2) A Web service is a software system designed to support interoperable
machine-to-machine interaction over a network. It 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
-- Web Services Architecture (W3C Working Group Note 11 February 2004) (http://www.w3.org/TR/ws-arch/)