Service-Oriented Architectures aren't new. The underlying ideas of encapsulation, location independence, and programming-by-contract have been in use since it was apparent that dependencies between components (processes, objects, and the like) should be minimized. SOA is the most recent incarnation of these concepts in the current technologies.

Like other attempts to support software intercommunication on a large scale, SOA has its strengths and weaknesses. It must overcome obstacles of different technologies, communication protocols, and Quality-of-Service (QoS). If you're familiar with existing concepts of distributed enterprise computing (such as CORBA or DCOM), the elements composing SOA architecture have clear analogies to the older technologies.

In short, SOA is a means by which a distributed, heterogeneous grouping of hardware and software can exchange information to satisfy a business goal.