It is built on top of the Qt , C ++ framework. It is quite advanced and has some good tools like Qt Designer for developing your applications. You should know that this is not like Python 100%, but next to it.
This structure is really good. It is actively developed by Trolltech, which is owned by Nokia. Binding for Python developed by Riverbank.
Nokia has announced that they will start using LGPL to launch Qt-Framework with Qt 4.5 (coming out in April, I think), but he is not sure yet that Riverbank is following this and is releasing Python bindings for LGPL too. (They have a commercial and GPL license at the moment.)
Qt is not only a GUI structure, but many other classes can also be created by simply using Qt classes. (Like SQL, networking ...)
Qt does not use its own GUI elements, but Wikipedia mentions that in recent versions of Qt uses its own widgets , where possible. I did not find evidence in the documentation, but for Mac OS X.
wxPython is a binding for Python using wxWidgets -Framework. This structure is under the LGPL license and is developed by the community of sources.
What I really miss is a good tool for developing an interface, they have about 3, but none of them are applicable.
One thing I should mention is that I found a tab error, despite the fact that I did not use advanced. (Mac OS X only) I think wxWidgets is not as polished as Qt .
wxPython is really just about GUI classes, there aren't many.
wxWidgets uses its own GUI elements.
Other
I have no experience with other GUIs, maybe someone else has.