Difference between JAX-WS, Axis2 and CXF

What is the difference between:

1. JAX-WS
2. Axis2
3. CXF

All three can be used to create webservices in Java.
As of I know JAX-WS is a specification and Axis2 and CXF are implementations, but Java 1.6 has implementation of JAX-WS if I am not wrong.

So one can use Java 1.6 to develop JAX-WS web services without using Axis2 or CXF? Then what is the use of Axis2, CXF?