This is the root category of Puzzles Wiki. Every other category can be found from here.
Ideally, every category other than this one should be a subcategory of at least one other category, and every article should be in at least one category as well. A full list of existing categories can be found at Special:Categories.
To add new categories, just add [[category:categoryname]] to the article (or any other page) you're trying to categorise. Just make sure that you check whether a suitable category already exists before creating a new one.