The acronym Tix stands for Tk Interface Extension. Tix is different things for different people.
If you are a GUI application programmer, that is, if you earn a living by building graphical applications, you will appreciate Tix as a library of mega-widgets: widgets made out of other widgets. To use a crude analogy, if the widgets in the standard TK library are bricks and mortars for a builder, the mega-widgets in the Tix library are walls, windows or even pre-build kitchens. Of course, these ``bigger components'' are themselves made of bricks and mortars, but it will take much less effort to put them together than planting bricks on top of each other.
The Tix widgets not only help you speed up the development of your applications, they also help you in the design process. Since the standard Tk widgets are too primitive, they force you to think of your house as, by using the same analogy, millions of bricks. With the help of the Tix mega-widgets, you can design your application is a more structural and coherent manner.
Moreover, the Tix library provides a rich set of widgets. Figure 1-1 shows all Tix widgets - there are more than 40 of them! Although the standard Tk library has many useful widgets, they are far from complete. The Tix library provides most of the commonly needed widgets that are missing from standard Tk: FileSelectBox, ComboBox, Control (a.k.a. SpinBox) and an assortment of scroll-able widgets. Tix also includes many more widgets that are generally useful in a wide range of applications: NoteBook, FileEntry, PanedWindow, MDIWindow, etc.
With all these new widgets, you can introduce new interaction techniques into applications, creating more useful and more intuitive user interfaces. You can design your application by choosing the most appropriate widgets to match the special needs of your application and users.