SAP GUI
SAP GUI is the active graphical user interface of the SAP system. It is also referred to as the programme used to create the interface. The concept is designed in that the graphical user interface is presented by means of GUI. It is thus the last component of a three-tier architecture in the multi-level client-server model.
The GUI program is the gateway to one or more application servers. Among them are the SAP Web AS ABAP or SAP ERP. In order to cover several operating systems, SAP has developed various implementations of SAP GUI.
More information about SAP operation can be found here.