A web service is an open programmatic interface that you can use to complete a standard UI task or action without accessing the UI. It can be expressed and accessed by using XML (EXtensible Markup Language) and XML messaging.
A web service is a modular enterprise application that can be published, discovered, and started dynamically across the web. Open, XML-based standards and transport protocols are used to achieve data transfer and integration between different applications. Web services are independent of the platform and the language.
The web services are defined in Java™ and the web service types are defined in WSDL/ XML schema language. The syntax of each web service is defined in XML schema language to allow a more explicit description of properties than is possible with Java. For example, using XML you can specify minimum and maximum entry length of attributes.
Web services are used for functions that can vary from simple requests to complicated business processes. After a web service is deployed, a developer of a client application can learn that the web service exists, what the web service can be used for, and how to interact with it. This process is known as web service discovery.