What is service-oriented architecture?

SOA is a way to design a complete solution, it is a set of commonly accepted practices for communication, state management, compatibility, etc. In software architecture specifically, SOA is a set of services (not necessarily Web Services) that are built independently to support a range of client applications. The modular design helps maintenance, business collaboration. SOA also provides some guidelines for development:

• Constraints over backward compatibility
• Metadata exposure
• Discoverability of services

On the other hand, WCF is just a supporting technology that helps you build the services in .NET.

You can create a SOA without WCF, just as creating a bunch of WCF services does not make your architecture a service oriented one.