Postnuke is a weblog based on the PHP Nuke source tree.
The author of PHP Nuke has a closed development model that some people to not agree with, or do not want, since it is hard to get features implemented.
Postnuke, in contrast, is hosted on Sourceforge and has an open development model, users are able to submit feature requests via the regular sourceforge trackers.
Postnuke and Phpnuke share alot of the same features, the administrational interface, the user interface, and the poor theme support. (Poor theme support because you have to know a fair bit of php to be able to write entirely original themes.)
Although the visual difference is minimal, the code difference is quite a bit, Postnuke has been rebuilt in a modular format, which is supposed to improve speed, and the ability to modify it.
Sources:
A comparison of Postnuke and PHPnuke can be found at linux.com: http://linux.com/newsitem.phtml?sid=1&aid=12497
The postnuke website can be found at http://www.postnuke.org