Essentially, Web service technology is to Web service clients and Web services what the world wide telephone system is to the parties that use telephone lines. The world wide phone system provides a standard way for different types of phones and other devices to communicate and also provides a standard way for parties to locate and call each other on the phone system. As long as a caller knows a few basic facts about the party they are trying to call, such as the phone number, language, and identity (or function) of the party, the caller can establish virtually immediate contact to tell (or send) the party information and to receive replies from them, no matter where in the world the two parties are connected to the phone system. This works for all compatible parties on the phone system, whether human or machine. In fact, both the number of parties and the uses that they make of the phone system have increased steadily over time, including such innovations as e-mail and faxes, as well as the original voice communications for which it was intended. It all works because of the agreed-upon standards developed in support of the basic telephone system.
For computer applications with access to a network, the same thing essentially applies to Web services. As long as two applications agree on (or can translate) the protocols, data types, and operations used to talk with each other, one such application (acting as client) can tell the other application to perform some function on its behalf or can request information from the other application, no matter where the two applications are located on the network or what execution environment they are running in. Like the phones, faxes, modems, other devices, and their users in the telephone system, the number and types of applications for Web services is potentially limitless.
