MDSN has a tutorial on service-oriented architectures (SOA):
It’s would be easy to conclude that the move to Service Orientation really commenced with Web servicesabout three years ago. However, Web services were merely a step along a much longer road. The notion of a service is an integral part of component thinking, and it is clear that distributed architectures were early attempts to implement service-oriented architecture. What’s important to recognize is that Web services are part of the wider picture that is SOA. The Web service is the programmatic interface to a capability that is in conformance with WSnn protocols. So Web services provide us with certain architectural characteristics and benefitsspecifically platform independence, loose coupling, self description, and discoveryand they can enable a formal separation between the provider and consumer because of the formality of the interface.
Service is the important concept. Web Services are the set of protocols by which Services can be published, discovered and used in a technology neutral, standard form.
In fact Web services are not a mandatory component of a SOA, although increasingly they will become so. SOA is potentially much wider in its scope than simply defining service implementation, addressing the quality of the service from the perspective of the provider and the consumer. You can draw a parallel with CBD and component technologies. COM and UML component packaging address components from the technology perspective, but CBD, or indeed Component-Based Software Engineering (CBSE), is the discipline by which you ensure you are building components that are aligned with the business. In the same way, Web services are purely the implementation. SOA is the approach, not just the service equivalent of a UML component packaging diagram.
SOA is not just an architecture of services seen from a technology perspective, but the policies, practices, and frameworks by which we ensure the right services are provided and consumed.