Role Based Access Control (RBAC) is the new permissions model in Microsoft Exchange Server 2010. With RBAC, you don’t need to modify and manage access control lists (ACLs), which was done in Exchange Server 2007 and earlier. On the flip side – as with anything new, RBAC can seem a bit intimidating at first.

I am going to try an explain how to think about RBAC, and the order to create things in so that you end up with a working RBAC setup that does exactly what you want….

        Continued at source (You Had Me at EHLO)….. RBAC explanation at MS Exchange team blog