Until the release of Dynamics NAV 2009 there was only one client available on the Windows platform. After the release of the Dynamics NAV 2009, Microsoft introduced the Role Tailored Client. Following this the old client is referred to as the Dynamics NAV Classic Client.
The main differences between the two is that the classic client is based on the traditional two-tier client/server concept, and the new client on a three tier concept:
It is also possible to use the Classic Client to run FDB files in single user mode without a Database Server.
It has long been known that Microsoft plans to retire the Classic Client completely. Still with the release of NAV 2009 R2 there is no way to run Dynamics NAV without the Classic Client as this is still the only place where you can add users, do database maintenance and use the development environment. But it's expected that after the release of the next major version (NAV "7.0"), then the Classic client will be retired and no longer released.