While there is a lively debate and history behind PHPNuke and its abilities. I've found the forked version of PostNuke based on an earlier version of PHPNuke to be better organized and straightforward. They are CMSs (Content Management Systems) and there are many options and alternative available on the web.
These projects are open-sourced software that run on a macosx/linux/unix/windows server that integrate the PHP (pre-hypertext processor) and MySQL databases to allow for dynamic websites with abilities to post new content into the database and have it follow the templates provided.
When fully installed and configured there are many modules and add-ons that allow for sophisticated community interaction such as forums, calendars, photo-image libraries, commenting of news, photos and articles.
In addition to PHPNuke, and PostNuke, there are Xoop, and EzPublish and others. You can search for them at:
<a href="http://sourceforge.net/" target="_blank">http://sourceforge.net/</a>