Service Oriented Architecture (SOA) is not a single program or technology. It's really a matter of software architecture or design. Some of the basic principles of SOA are:
- Based on inter-operable services: Instead of building an application from a huge monolithic program, SOA design calls for breaking systems into multiple services. These services are not rigidly coded together, but interact by sending and receiving messages.
- Software reuse: By dividing large applications into services, individual services, or groups of services can be reused.
- Loose coupling: Services are not rigidly coded together, but rather interact through "loose coupling" where services communicate by sending and receiving messages.
- Abstraction: The messages sent between services follow a well-defined contract. No information on the internal implementation of the services is needed.
- Location: Clients and services don't store the network or server location of target services. The information is made discoverable in a registry.
In hardware terms, a "bus" is a physical connector that ties together multiple systems or subsystems. Instead of having a large number of point-to-point connectors between pairs of systems, you connect each system to the bus once.
An Enterprise Service Bus (ESB) does the same thing, logically, in software. Instead of passing electric current or data over the bus to and from the connections (or "endpoints") on the ESB, the ESB logically sits in the architectural layer above a messaging system. The messaging system allows for asynchronous communications between services over the ESB. In fact, when you are working with an ESB, everything is either a service (which in this context is your application software) or a message being sent between services. It's important to note that a "service" is not automatically a web service. Other types of applications, using transports such as FTP or JMS, can also be services.
For more information on what an ESB is, a great resource is Enterprise Service Bus: Theory in Practice by David Chappell.