Service-oriented architecture (SOA)

SOA is an architectural style whose goal is to achieve loose coupling among interacting software agents. A service is a unit of work done by a service provider to achieve desired end results for a service consumer. Both provider and consumer are roles played by software agents on behalf of their owners.
SOA-based systems inter-operate based on a formal definition (or contract, e.g., WSDL) that is independent of the underlying platform (such as Java, .NET etc) and programming language (like. C#, Java). Services written in C# running on .NET platforms and services written in Java running on Java EE platforms, for example, can both be consumed by a common composite application. Applications running on either platform can also consume services running on the other as Web services, which facilitates reuse.
For more information on the topic checkout following references.

Reference:
http://en.wikipedia.org/wiki/Service-oriented_architecture
http://webservices.xml.com/pub/a/ws/2003/09/30/soa.html