SOA is an architectural style for building software applications that use services available in a network such as the web. It promotes loose coupling between software components so that they can be reused. Applications in SOA are built based on services. A service is an implementation of a well-defined business functionality, and such services can then be consumed by clients in different applications or business processes. 

SOA allows for the reuse of existing assets where new services can be created from an existing IT infrastructure of systems. In other words, it enables businesses to leverage existing investments by allowing them to reuse existing applications, and promises interoperability between heterogeneous applications and technologies. SOA provides a level of flexibility that wasn't possible before in the sense that: 

Services are software components with well-defined interfaces that are implementation-independent. An important aspect of SOA is the separation of the service interface (the what) from its implementation (the how). Such services are consumed by clients that are not concerned with how these services will execute their requests. 
Services are self-contained (perform predetermined tasks) and loosely coupled (for independence) 
Services can be dynamically discovered 
Composite services can be built from aggregates of other services

http://kalitinterviewquestions.blogspot.com/2009/06/what-is-serivice-oriented-architecture.html