This may
include menus to start programs, trays or panels to indicate currently running
programs, icons that represent files or programs on the desktop background,
docked applets, and other useful tools and utilities.
Application clients
Programs that enable the user to perform useful work. They are spreadsheets,
word processors, web browsers, media players, video editors, and so forth.
Toolkits
Programming libraries that are used to simplify the task of writing clients that
communicate with an X server. Toolkits are not a layer per se, but they do support
and simplify the construction of the client layers.
The software used in any layer can be changed without affecting the other layers. For
example, you can switch from the XDM display manager to the GDM display manager
without making any changes to the other layers.
The bottom two layers (Network Transport and X Server) are mandatory; the other
layers are optional. This provides a lot of flexibility in the way that the GUI operates.
For example, the user of an automated teller machine doesn??™t need to log in with a
user ID, to move or resize windows, or to manage files and start programs, so the
display manager, window manager, and desktop environment layers are not needed;
the ATM application can directly take control of the entire display space.
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