In the most simplistic way a Web service may be defined as "a Web accessable program code which performs a task of either processing or serving some data. Although there are many other definitions in the related literature, the one in W3C (2004) seems to be quite complete and refering to also lately popular REST style Web services. It states that " We can identify two major classes of Web services: REST-compliant Web services, in which the primary purpose of the service is to manipulate XML representations of Web resources using a uniform set of "stateless" operations; and arbitrary Web services, in which the service may expose an arbitrary set of operations.