[[ ... relation to Python, Tcl/Tk, ...]]
[virtues: easy start, portability]
Installation: Tkinter is usually part of Python. However, Tcl/Tk often is not. For information on getting Tcl/Tk/Tkinter installed on your platform, refer to How to install Tkinter.
Several books mention Tkinter, and a handful of mailing lists are likely to be useful to those with an interest in Tkinter.
References on the Web:
Stephen Ferg's Thinking in Tkinter,
Bruce Embry's Python Course with 6 lessons on Tkinter,
Ancestral pages on the Python Wiki covering Tkinter and How Tkinter can exploit Tcl/Tk extensions,
Tkinter module API via epydoc. ( <-- Is this still around or can another link be found? )
http://www.tkdocs.com/ is a site about Tk which now includes also python3+Tkinter examples
- and ...
Tk includes extensive online help, in the ManPage format on UnixOperatingSystems. A standard ActiveTcl installation on Windows also has all the same information in Windows help files. The information there is useful to Tkinter programmers, but you need to know how to TranslateTcl to Python/Tkinter first. In all of this, though, it's important to understand that there is no comprehensive and coherent reference, in the sense of a single location to learn about all options and functions of, say, the canvas widget. Developers typically learn the basics in a tutorial, inspect Tcl/Tk documentation to learn about all the attributes likely operative, read one of the books for advanced usage, and so on.
[comparisons]
License: Tcl/Tk is distributed under the BSD license, and Tkinter under the Python license. This means they can be used in proprietary applications. Also, the FSF has determined that the BSD and Python licenses are GPL compatible.
At another level, "Tkinter is a shared library that links to the Tcl & Tk shared libraries", in Jim Ingham's words. The Python interpreter loads (this) Tkinter in as a "shim" that exposes Tcl/Tk commands within Python.