There are some key differences between Web services and SOA.
Web services define a web technology that can be used to build applications that can send /receive messages using SOPA over HTTP. However, SOA is an architectural model for implementing loosely coupled service based applications.
Web services can be used to implement SOA applications. Even though web service approach to SOA has become very popular, it is only a single method of implementing SOA.
SOA can be implemented using any other service-based technology (e.g. CORBA and REST).
