Service-oriented architecture (SOA) is an approach to defining integration architectures based on the concept of a service. The services provide business and infrastructure functions, and form the building blocks of an enterprise.
Users request functions from business services, in language familiar to the user, and without any need to know how or where those services are provided. Business services can also request functions from other business services. [More from p27]
Services are provided by service providers.
Service requesters invoke services without knowing where the service provider exists.
Services can be invoked independently by service requesters to process simple functions, or can work together by choreographic implementations to quickly devise new functionality to existing processes.
A service infrastructure implements the connection between a service requester and service provider.
Business services leverage application and infrastructure services
The enterprise service bus is a service-oriented infrastructure that implements the SOA principles in a scalable, manageable way in a heterogeneous world.