See Figure 4 on page .
Sometimes an application may be tiered in at least three distinct pieces. One piece is the presentation or graphical user interface(GUI). Another piece is the logic, functions, procedures, and objects that materialize this particular application. Well another piece is the data management most often materialized by a database.
The three tiered architecture allows for one central server location for all the business logic and one central server location for all of the data leading to reuse, consistency, and uniformity of applications in this environment.
The presentation layer supports the GUI. Sometimes this is called a GUI-lite application. There may be many different kinds of GUI.
The middle section materializes the application. Though this is called the application logic it is really much larger then just logic formulas and includes all function and procedures that make the application an application for a particular domain. Many of the elements in this area are tailored for the application needs. Sometimes this layer is called the business rules or logic.
The last layer is the data. This is most often materialized by a database. This layer does not have to be limited to a database. Alternatively, a large computational server could act as a mathematical engine.
The three tiered layering is popular because it forces clean lines between the three layers. The ``thin'' client can easily be moved to other architectures. The application logic can be used by other applications in the system. The database layer allows for plug and play of many different database vendors.
Each layer may have a special interface or use a more generic interface protocol. An example of a GUI protocol is xterm or html. An example of a database protocol is SQL/Net.