Why does Wikia use a heavily modified version of MediaWiki?

To correctly understand the answer to this question, we'll have to talk about how MediaWiki actually works. First, there is what we call "MediaWiki Core".  It is the most basic form of the wiki software and is (relatively) featureless compared to what you see on Wikia or WIkipedia.  It is the "tarball" release of the software. Second, we have what we call "Extensions".  Extensions are add-ons or plug-ins that add, modify, or remove functionality from the core MediaWiki install.  Extensions can do all sorts of things, from adding rating tools to social media plug-ins to, well, anything. Third, we have "Skins".  These are most easily understood as the user interface elements - how things lay out, colors, etc.  Skins can also add, hide, or modify the way the site works. So.  That being said, neither Wikia nor Wikipedia run a "stock" MediaWiki.  The reason that Wikia does not run a "Wikipedia-alike" is because the users of Wikia have different needs than those of Wikipedia (or even other MediaWiki sites, like WikiHow). You can look at the MediaWiki versions and installed extensions of both sites here: http://www.wikia.com/Special:Version http://en.wikipedia.org/wiki/Special:Version