Tkinter is Python's de-facto standard GUI (Graphical User Interface) package. It is a thin object-oriented layer on top of Tcl/Tk.
Tkinter is not the only GuiProgramming toolkit for Python. It is however the most commonly used one, and almost the only one that is portable between Unix, Mac and Windows. CameronLaird calls the yearly decision to keep TkInter "one of the minor traditions of the Python world."
More info here: http://www.python.org/topics/tkinter/
MythDebunking: TkInter is ugly on Windows.