Web services are a standardized way for one application to invoke a method of another application. These applications may reside on different computers that are some way connected, such as by a local area network or, more commonly, by the Internet. With Web services you have two actors, a client and a server. The server makes some set of methods available. These methods, when called, will perform some action and/or return some data. The client invokes one of the available methods of the server. Like methods in a class library, the methods of a Web service can accept an arbitrary number of input parameters, and can optionally return a result. The Web services standard spells out in great detail how a client can invoke a Web service method from a server. The standard dictates how input parameters and return values are passed from one computer to the other, how faults are handled, and a myriad of other complications.
Before we delve into the specifics of the Web services standards, let's first think about what, exactly, a "service" is, and what fundamental properties are inherent to services.
