wxWindows is a cross platform library written in C++ that makes it possible to write a GUI program to a single API that runs on Windows, Unix/X, and Macintosh. wxPython is a Python extension module that wraps wxWindows for Python. It is quickly gaining popularity among Python developers and is the second most popular GUI toolkit for Python.
Because of some legal funkiness, wxWindows is now known as wxWidgets. I believe money was exchanged, so it wasn't all bad.