Web services are self-contained, modular applications that can be published, located, and invoked across the Web from any location, allowing you to transcend hardware and operating system boundaries. An implementation of SOA, Web services use a standard interface technology to make the services available.
Web service applications perform discrete functions ranging anywhere from simple requests to complicated business processes that combine information from multiple sources. Web services can be developed and componentized internally, or brought in and reused from the outside. To create a robust business solution, you assemble Web services like building blocks into a cohesive entity, mixing and matching software components as you need.
Conceptually, Web services are similar to any application services. What makes them unique is that they describe themselves to the outside world, revealing what functions they perform, how they can be accessed, and what kinds of data they require. Today, this is accomplished with the following standards: