Wiki/mediawiki/extensions
For most things you'd like to change that isn't one of the settings that MediaWiki comes with, you'll need to find an extension or make your own.
DO NOT HACK TO CORE
If you modify the system in certain ways, it will break site's ability to update in the future.
If you absolutely NEED to modify the core, use includes and imports to keep your changes separate from the original build.
That way, if an update is broken; you can revert the files to the original state (before you tinkered), update, then merge your changes back in.
Use Extensions
There's a decent community which maintains it's plugins. They're far more likely to "just work" after an update than core hacks.
If your plugin doesn't "just work" after an update, it's likely that there's an updated version that will work with your new version of MediaWiki.
