CBRS is a service that is provided during periods of intensity or episodes of need. There is a point in the provision of any rehabilitation treatment plan, in which the rehabilitation services being provided are no longer restorative in nature. A Member discharged from CBRS does not exclude the possibility that he or she may again
Another way to look at it is that the undergraduate course is aimed at training you how to use a tool (Probability for an example) while the graduate course is aimed at giveing you the foundation (theory) to develop your own tools.
This answer is a bit different from the others. The other answers deal with what a postdoc is. My answer describes the difference between the roles of grad students and postdocs in two ways. The first distinction is this: A graduate student is an apprentice, while a postdoc is a journeyman.