You've certainly come across the term
Web services before, although you may not be aware of what they are or how they fit into the way the Web operates—and will operate in the future. Suffice it to say that Web services provide the foundation of the new generation of Web applications. Whatever the client application is—whether it is a Windows application or an ASP.NET Web Forms application—and whatever operating system the client is running—Windows, Pocket Windows, or some other OS—they will regularly communicate over the Internet using a Web service.
Web services are server-side programs that listen for messages from client applications and return specific information. This information may come from the Web service itself, from other components in the same domain, or from other Web services. While the concept of the Web service is evolving continuously, there are several different types of Web services that carry out different functions: Some provide information specific to a particular industry such as manufacturing or healthcare; there are portal services that use services from different providers to offer information on a specific theme; there are services specific to single applications, and building block services that can be used by many different applications.
Web services give you the capability to combine, share, exchange, or plug in separate services from various vendors and developers to form entirely new services or custom applications created ...