Link - Reference Model for Service Oriented Architecture v1.0
The spec for the SOA Reference Model includes a very good discussion of the differences between the SOA approach and an OO ( "Object-Oriented" ) approach to web services, although strictly SOA is not about web services. In their definition, SOA is "a paradigm for organizing and utilizing distributed capabilities that may be under the control of different ownership domains". Sounds a bit vague to me, but it may be the clearest definition to date.