From the PyObjC homepage (http://pyobjc.sourceforge.net/):
The PyObjC project aims to provide a bridge between the Python and Objective-C programming languages. The bridge is intended to be fully bidirectional, allowing the Python programmer to take full advantage of the power provided by various Objective-C based toolkits and the Objective-C programmer transparent access to Python based functionality.
The most important usage of this is writing Cocoa GUI applications on Mac OS X in pure Python. See our tutorial for an example of this.