A. Grimshaw and S. Tuecke, "Grid Services Extend Web Services," Preprint ANL/MCS-P1036-0303, March 2003. [pdf]
We are often asked by people who are trying to understand the value Grid technology brings to Web services, "what is the significance of Grid services? They look like Web services." The answer to this question is superficially straightforward: mechanically, the differences between Grid services (as defined in the Grid service specification) and Web services are few: to first order, a Grid service is simply a Web service that conforms to a particular set of conventions. For example, Grid services are defined in terms of standard WSDL (Web Services Definition Language) with minor extensions, and exploit standard Web service binding technologies such as SOAP (Simple Object Access Protocol) and WS-Security (Web Services Security). So yes, Grid services do look like Web services, etc.