Part of the standard Python distribution. Documented in the Grayson and Lundh books: see the Python Library Reference manual: http://www.python.org/doc/current/lib/tkinter.html
Implemented through Tcl scripting interface, and the C API.
Gustavo Cordero has worked on a recipe to make some Tk extensions--tkhtml and tktable--available to Tkinter developers. These can be found in the Python/ directory of Tixapps.
More on this subject appears in the Python Wiki [L1 ].
The Python Wiki has its own page for Tkinter [L2 ].