Service-Oriented Architecture (SOA) is an architectural style that supports service orientation. Service orientation is a way of thinking in terms of services and service–based development and the outcomes of services.

A Service

• Is a logical representation of a repeatable business activity that has a specified outcome 
  (e.g., check customer credit; provide weather data, consolidate drilling reports)
• Is self–contained
• May be composed of other services
• Is a "black box" to consumers of the service

The use of services, when compared to bespoke applications provides major benefits.

• In contrast to the use of large applications, which tend to be "information silos" that cannot readily exchange information with each other, the use of finer-grained software services gives freer information flow within and between enterprises. Integrating major applications is often expensive. SOA can save integration costs.
• Organizing internal software as services makes it easier to expose its functionality externally. This leads to increased visibility that can have business value, as for example when a logistics company makes the tracking of shipments visible to its customers, increasing customer satisfaction and reducing the costly overhead of status enquiries.
• Business processes are often dependent on their supporting software. It can be hard to change large, monolithic programs. This can make it difficult to change the business processes to meet new requirements (arising, for example, from changes in legislation) or to take advantage of new business opportunities. Service-based software architecture is easier to change – it has greater organizational flexibility, enabling it to avoid penalties and reap commercial advantage. This is one of the ways in which SOA can make an enterprise more "agile".
• Using existing software modules rather than writing new ones means lower development and testing costs and – in many cases an even greater saving – lower maintenance costs.
• Application of management policy, for example by restricting a service consumer to a contracted service volume, or giving priority to certain kinds of message;
• Application of security policy, for example by controlling access to certain services, or rejecting messages that could damage the enterprise systems or the enterprise itself (for example messages containing viruses that could destroy data).
• Data translation – the conversion of data from one format to another through automated field mapping. Data conversion by specially–written software is expensive. The use of generic data translators can bring significant cost saving.
• Simplification of software structure – by removing functionality that is not business–related from the business software services
• Ability to adapt quickly to different external environments – by concentrating in one place the logic that relates environmental events to business events
• Improved manageability and security – when used with performance measurement and security event detection.
• When an enterprise adopts a service–oriented architecture, asset wrapping is typically applied to existing application software packages, which means that the value of an enterprise’s existing assets is preserved, the cost of developing or acquiring replacements is avoided, and there is a smooth migration path from the old architecture to the new one.
• Improved reliability – through redundant operation of the underlying assets, so that one can take over when another fails or is withdrawn for maintenance; and
• Ability to scale operations to meet different demand levels – through dynamically increasing or reducing the number of underlying assets that support a real asset, as demand rises and falls.