If you've ever been to Wikipedia, then you've seen Wikimedia, as it was written for Wikipedia.
https://www.mediawiki.org/wiki/MediaWiki (https://www.mediawiki.org/wiki/MediaWiki)
Many companies use it to put together the operations manual for their company, and delicate various sections to different people to collaborate on. Others use it to put together a historical encyclopedia of a certain item. MoparWiki is an example of that. http://moparwiki.com/index.php?title=Main_Page (http://moparwiki.com/index.php?title=Main_Page)
Wikis have a little Complexity to them because most use a formatting language. It is easy to pick up, but will take a little practice. Because Media was written to control the World's Largest Encyclopedia, the installation and upgrades are complicated and best left to pros. Even configuration (if you want to customize) is best kept to the pros. http://www.bigkahunahosting.com/hosting-services/ (http://www.bigkahunahosting.com/hosting-services/)
MediaWiki will use one Database.