WCF is a programming model and API. "WCF Service" implies an app that is built using that programming model and API.

"Web Service" is an app that exposes an HTTP (REST (XML or JSON), SOAP or otherwise) interface.

You can build a Web service using WCF, but you can also build a Web service using other APIs or "stacks". Like PHP or Java, for example.

With WCF you can build web services but you can also build services that are not, "Webbish". For example you can build a service that accepts incoming binary requests over only a local pipe interface. It is still a service, but it is not a "web service" because it is not using web protocols (generally HTTP and XML).