Web Services combine the best of both distributed componentization and the World Wide Web. It extends distributed computing to broader ranges of client applications. The best thing is that it does it by seamlessly marrying and enhancing existing technologies.
Web Services are distributed software components that are accessible through standard web protocols. The first part of that definition is similar to that of COM/DCOM components. However, it is the second part that distinguishes Web Services from the crowd. Web Services enable software to interoperate with a much broader range of clients. While COM-aware clients can understand only COM components, Web Services can be consumed by any application that understands how to parse an XML-formatted stream transmitted through HTTP channels. XML is the key technology used in Web Services and is used in the following areas of the Microsoft .NET Web Services framework:
The technology enabling universal understanding of how to perform data exchanges between the service provider and consumer; the format of data for the request and response.
The language describing how the service can be used. Think of this as the instructions on the washing machine at the laundromat telling you where to put quarters, what buttons to push, etc.
The process of advertising or publishing a piece of software ...