This category contains all categories marked as hidden. The categories that should appear here are the maintenance categories, that is, categories reflecting the present status of the page, rather than classifying the article subject.
To hide a category, uses the magic word
__HIDDENCAT__ - though it is preferable to use Template:Hidden category instead of the magic word directly. This also places the page in this category.
A logged-in user may elect to view all hidden categories, by checking "Show Hidden Categories" on the "Misc" tab of My Preferences. Notice that "hidden" parent categories are never in fact hidden on category pages (although they are listed separately).