Unlike SOAP and the WS- stack, which are specified as W3C standards, REST is really a set of principles for designing and using web-based interface. REST / RESTful applications rely heavily on other standards:
HTTP URI, URL XML, JSON, HTML, GIF, JPEG, and so forth (resource representations)
The role of JAX-RS (Java API for RESTful Web Services) is to provide APIs that support building RESTful services. However, JAX-RS is just one way of doing this. RESTful services can be implemented other ways in Java, and (indeed) in many other programming languages.