Web Services provide loosely coupled communication between enterprise systems.
Web Services also enables the enterprise systems to expose business functions to Web-enabled clients.
Web Services can be accessed by wider variety of Clients.
Web Services provide Security and quality services such as availability, reliability and transactions.
There are two principle models in web services. These are:
Document oriented Model.
Remote procedure call oriented Model.
Web Services can be implemented by two ways
Synchronous Web Services
Asynchronous Web Services.
Web Services should be used when you need interoperability across heterogeneous platforms. That is, When you need to expose all or part of your application to other applications on different platforms.
Implementing Web Service make sense when your web services clients are potentially non J2EE enterprise applications.