Graphical desktop application programming interfaces

Desktop toolkits for X11-compatible X-servers (GTK, QT)

For contemporary and fast Linux GUI programming, one usually uses a high-level C/C++ programming toolkit library like QT or GTK. For programming languages that come with their own UI toolkits (like Java), one even doesn't have to care about the platform - those languages follow the paradigm "write once, run everywhere". However, in practice you will notice small differences between the platforms, even when the paradigm mentioned should apply.

Classic X11-API GUI programming

The "X Window System" (X11) is window system, which was first developed at MIT in 1984, then it was enhanced by various non-profit organzations like XFree86 and X.Org over the years. On the Debian clients managed by ISG D-ITET, "X.Org" (X server) is the current reference software implementation based the original X11 server. Most programmers nowadays no longer directly write software that is interacting with the X server server directly (for those who are still interested in that, read: X11 Programming).