This is the top-level category, or looking at it another way, the root category. Every page in the wiki (excluding user pages) is intended to be reachable from here.
The Wiki Organization category is used to store all templates and project pages of the wiki; all other pages contain content related to the Diablo universe in one way or another. A full list of existing categories can be found at Special:Categories.
To add a new category to a page, just add [[Category:Categoryname]] to the end of the article or existing category page.
This category has the following 17 subcategories, out of 17 total.