A service-oriented architecture (SOA) is an architectural pattern in computer software design in which application components provide services to other components via a communications protocol, typically over a network. The principles of service-orientation are independent of any vendor, product or technology.
A service is a self-contained unit of functionality, such as retrieving an online account information. By that definition, a service is a discretely performed operation. However, in the Web Services Definition Language (WSDL), a service is an interface definition that may list several discrete services/operations. And elsewhere, the term service is used for a component that is encapsulated behind an interface. This widespread ambiguity is reflected in what follows.
Services can be combined to provide the functionality of a large software application. Service Oriented Architecture makes it easier for software components on computers connected over a network to cooperate. Every computer can run any number of services, and each service is built in a way that ensures that the service can exchange information with any other service in the network without human interaction and without the need to make changes to the underlying program itself.