SOA: What and Why
A service oriented architecture (SOA) consists of distributed services that can dynamically interoperate.
A service performs a well defined function with a well defined interface, increasingly Web services. It is an application building block that implements a business function such as order entry or a shared service such as data transformation.
Business processes are implemented by orchestrating services, and then quickly re-orchestrating them to meet changing requirements, without changing the code of the application building blocks.