Web Service can be thought of as a combination of a web site and a windows service. It exposes some functionality over a web.
For example you have a library and you write a function to display the list of books and you expose this as a web service, now anyone can use this function in his application to display this list of books.
So the functionality written by you can be reused by developers and the results can be displayed in the format he wants.
Web Services can convert your application into a Web-application, which can publish its function or message to the rest of the world.Web services allow different applications from different sources to communicate with each other without time-consuming custom coding, and because all communication is in XML, Web services are not tied to any one operating system or programming language.For example,Java can talk with Perl, Windows applications can talk with UNIX applications.Unlike traditional client/server models, such as a Web server/Web page system, Web services do not provide the user with a GUI. Web services instead share business logic, data and processes through a programmatic interface across a network.