The GNU Enterprise Application Server ('AppServer') is the core of the n-tier variant of the GNU Enterprise system. To the front end (be it GNUe Forms, GNUe Reports or any other tool), it provides user-defineable business objects with arbitary fields and methods. While transforming access to those fields and methods into database communication and calling of scripts, it cares about stability, security, speed, and consistency.
Because it uses the GNUe Common libraries, it can use any back-end database server and RPC (Remote Procedure Call) method that Common supports.
Diagrams of how the various parts of AppServer fit together are available.