The ttk widget is based on a revised and enhanced version of TIP #48 (http://tip.tcl.tk/48) specified style engine.
The basic idea is to separate, to the extent possible, the code implementing a widget's behavior from the code implementing its appearance. Widget class bindings are primarily responsible for maintaining the widget state and invoking callbacks, and all aspects of the widgets appearance lies under themes.
You can substitute some Tkinter widgets with their corresponding ttk widgets (Button, Checkbutton, Entry, Frame, Label, LabelFrame, Menubutton, PanedWindow, Radiobutton, Scale, and Scrollbar).
However, Tkinter and ttk widgets are not completely compatible. The main difference is ...