I configured a wiki recently, and wanted a way to categorize the articles in a hierarchical structure and display it in a visual manner (possibly with a category tree). I did a quick search in MediaWiki, but it's explanation was a bit confusing - unclear.
After some trial and error, I figured it out, so here is a quick tutorial on how to do exactly that.
Suppose this is the category structure we want to establish.
To assign Apples, Bananas and Oranges under the Fruits category, all we have to do is edit their individual pages, and add the category tag [[Category: Fruits]], we do the same for the individual pages that make up the Veggies category.
To assign the Fruits category and Veggies category to the Edibles category, we first have to find those categories. You can locate the Fruits category by going to http://mywiki.com/index.php/Category:Fruits
Once you've found the category page itself, you'll see it's nodes listed (i.e. Apples Bananas and Oranges). Now you have to edit this page, and assign it the Edibles category. Do the same for the Veggies category, and you're done.
This isn't tricky, but it's not straight forward either.
There is a very neat extension to display these categories in a tree structure. It's called CategoryTree. Simple to install, has a few configuration option and good section on troubleshooting potential problems.
With the CategoryTree extension, you should ask it to print out the Edibles category. Using that, you will see them listed in a tree/dropdown fashion.