WCF Webservice and REST Web services aint same. WCF WS may or may not be REST and REST may or may not be implemented using WCF. REST uses the http verbs (GET, DELETE, PUT, POST etc). To specify JSON as the format of data interchange, you need to add "RequestFormat=WebMessageFormat.Json, ResponseFormat=WebMessageFormat.Json" in the WebInvoke attribute.
Please refer to the article -
WCF REST Service with JSON for more details.