The standard graphical subsystem for UNIX and Linux, called X,
has its own libraries for GUI development. They provide a
low-level programming interface to X, but tend to be hard to
use. Old end-user applications and other toolkits of course make
good use of them. Nowadays the Linux GUI scene is dominated by
GTK+ and Qt, since two popular, complete user environments - GNOME
and KDE - are based on them.
Whether the toolkit is suitable for a newbie programmer.
License
Different licenses for different GUI toolkits have practical
significance. GTK+, TK and GNUstep licenses allow you to develop both open
source and closed source applications without paying for a
license. Motif license requires payment, while the QT license
requires payment only if you write closed source programs.
Language
The language that is most often used with the toolkit.