Service-oriented Architecture (SOA) is an architectural style that transforms business use cases into a set of interlinked services or reusable business tasks that can be accessed over a network. This could be an intranet or over the Internet. The services could be geographically and technologically diverse. SOA can combine services hosted on remote locations as if they are hosted on your local machine, and accomplish a specific business task, enabling your business to quickly adapt to changing conditions and requirements.

Service is a self-contained unit of business tasks, such as a credit card payment or stock quote. SOA orchestrates the services to accomplish a bigger task. The main theme of SOA is loose coupling so that you can reuse the services, for instance, define fine-grained services and combine them in a coarse-grained service.

Organizations can have existing heterogeneous IT systems, such as a payroll system developed in C++ and an expense claim workflow developed in Java. SOA enables businesses to leverage existing investments, by allowing them to reuse existing IT systems, and accomplishes interoperability between heterogeneous applications and technologies.

For more details on SOA, you can visit http://www.oracle.com/technetwork/articles/javase/soa-142870.html or the book Applied SOA: Service-oriented Architecture and Design Strategies.

SOA can rely on web services for interoperability between heterogeneous applications and technologies. In the next section, we'll explore the web services.