The fat client model places more function on the client. Fat clients are the more traditional form of client/server. The bulk of the application runs on the client side of the equation. In both the file server and database server models, the clients know how the data is organized and stored on the serverside. Fat clients are used for decision support and personal software. The provide flexibility and opportunities for creating front-end tools that let end-users create their own applications.
The Fat server model places norm function on the server. Fat server applications are easier to manage and deploy on the network because most of the code runs on the servers. Fat servers try to minimize network interchanges by creating more abstract levels of service. Transaction and object servers, encapsulate the database. Instead of exporting raw data, they export the procedures that operate on the data. The client the eat server model provides the GUI and interacts with the servers through remote procedure calls.
Each client/server model has its uses. In many cases, the models complement each other and it is not unusual to have them coexist in one application. For example, a groupware imaging application could require an all-in-one server that combines file, database, transaction and object servers. Fat servers used for mission-critical applications, represent the new growth area for PC-Based client/server computing.