Un bell'articolo che introduce il concetto:
http://msdn.microsoft.com/msdnmag/issues/03/07/designpatterns/default.aspx
Qui riporto le prime righe del documento......
No man is an island. That's even true of developers. Software architects, designers, and developers need to communicate with each other in order to be successful. The same is true with applications. It used to be that an application could be thought of as self-contained. Mainframe applications, and on a smaller scale PC applications, contained everything they needed. They controlled the user interface, the business logic, and the data, all without having to look outside the application or letting outsiders in.
This isn't so anymore. The applications we build today often need to integrate with existing software, and that trend is likely to continue. What's more, increasingly applications are losing control of the user interface, both because it's designed elsewhere and because it's accessing the application over the Internet.
Obviously, when we architect and design applications, we should consider this need for integration across computing platforms. Luckily, there's an architectural solution. Service-Oriented Architecture (SOA) sees an application as a service that's discoverable and accessible over the Internet without regard to the platform the consumer is running on or which programming language was used in order to create the consumer. In other words, a service provides platform- and language-independent services. Web Services is just one example of this.