What is Web service ?

An application that exists in a distributed environment, such as the Internet. A Web service accepts a request, performs its function based on the request, and returns a response.

The request and the response can be part of the same operation, or they can occur separately, in which case the consumer does not need to wait for a response.

Both the request and the response usually take the form of XML, a portable data-interchange format, and are delivered over a wire protocol, such as HTTP.