Web services are defined in many different ways. The following definitions are from leading companies who are involved in Web services.
W3C: A Web service is a software application identified by a URI, whose interfaces and binding are capable of being defined, described, and discovered by XML artifacts and supports direct interactions with other software applications using XML-based messages via Internet-based protocols.
Microsoft: Web services provide the ability to exchange messages in a loosely coupled environment using standard protocols such as HTTP, XML, XSD, SOAP, and WSDL.
IBM: Web services are a new breed of Web application. They are self-contained, self-describing , modular applications that can be published, located, and invoked across the Web.
Rogue Wave: Applications that interoperate in a loosely coupled system using Internet protocols.
The Stencil Group: Loosely coupled software components that encapsulate discrete functionality and that are accessible over standard Internet protocols.
Sun Microsystems: A Web service is an application that exists in a distributed environment, such as the Internet. A Web service accepts a request, performs its function based on the request, and returns a response.