Most professional applications interact with users through a graphical user interface (GUI). A GUI is normally programmed through a toolkit, which is a library that implements controls (also known as widgets) that are visible objects such as buttons, labels, text entry fields, and menus. A GUI toolkit lets you compose controls into a coherent whole, display them on-screen, and interact with the user, receiving input via such devices as the keyboard and mouse.
Python gives you a choice among many GUI toolkits. Some are platform-specific, but most are cross-platform to different degrees, supporting at least Windows and Unix-like platforms, and often the Macintosh as well. Check http://phaseit.net/claird/comp.lang.python/python_GUI.html for a list of dozens of GUI toolkits available for Python. One package, anygui (http://anygui.org), lets you program simple GUIs to one common programming interface and deploy them with any of a variety of backends.
The most widespread Python GUI toolkit is Tkinter. Tkinter is an object-oriented Python wrapper around the cross-platform toolkit Tk, which is also used with other scripting languages such as Tcl (for which it was originally developed) and Perl. Tkinter, like the underlying Tcl/Tk, runs on Windows, Macintosh, and Unix-like platforms. Tkinter itself comes with standard Python distributions. On Windows, the standard Python distribution also includes the Tcl/Tk components needed to run Tkinter. On other platforms, you must obtain and install Tcl/Tk separately.
This chapter covers an essential subset of Tkinter, sufficient to build simple graphical frontends for Python applications. A richer introduction is available at http://www.pythonware.com/library/tkinter/introduction/.