The Role Tailored Architecture was introduced together with Dynamics NAV 2009 and is different from the Classic Architecture by it's three-tiered architecture with a database server, an application server or service tier and the client.
The database server is also called the data level of the three-tier model. With the NAV Role Tailored Architecture, then Microsoft SQL Server is the only database server which can be used.
The Dynamics NAV Service Tier is the handling the business logic and communication between the database server and the client.
The Dynamics NAV RoleTailored Client is the user's presentation level of the three-tier model. Microsoft has also announced that they will release a client which can be used directly from Microsoft Sharepoint. The What are Dynamics NAV/Navision Web Services? is another client/presentation level component.