What Are Web Services?
Simply put, Web Services allow applications that are running on any hardware platform, in any operating system, and written in any computer language, to invoke application logic and to exchange information with each other. This is possible because these applications have been configured to send and receive messages in a consistent format, such as XML. Web Services know how to construct XML that represents a service request of another application and how to construct XML that represents their internal data. In addition, Web Services can parse XML requests coming from other applications and invoke their underlying functionality in response to these requests.
Web Services provide the capability for systems to be loosely coupled and easily interoperate with other software systems, without having to go through weeks or months of integration efforts. Systems can discover each other dynamically through search type functionality (UDDI, discussed later) and negotiate their interactions automatically (WSDL, discussed later). The messages exchanged by the systems are XML-based messages with some wrapper information (SOAP, discussed later).