Web Services are self-contained modular applications that can be published and invoked over a network (such as the World Wide Web). Web Services provide well-defined interfaces that describe the services provided. You use Web Services to produce or consume programmable services over the Internet using emerging standards such as XML, XML Schema, SOAP (Simple Object Access Protocol), and WSDL (Web Service Definition Language). 

Web Services use SOAP, a standard lightweight protocol for exchanging information in a distributed environment. It uses HTTP as a communications protocol and XML to encode remote procedure calls. 

You can build servers to implement Web Services and clients that call on those services. You can write clients for arbitrary servers to implement Web Services that respond to SOAP messages, and servers to publish Web Services for use by arbitrary clients. 

Refer to Using Web Services for more information on Web Services.