Wikimedia Commons has media related to Hidden categories.

This is a maintenance category, used for maintenance of the Wikipedia project. It is not part of the encyclopedia and contains non-article pages, or groups articles by status rather than subject. Do not include this category in content categories.

This is a container category. Due to its scope, it should only contain subcategories.

This is a tracking category. It builds and maintains a list of pages primarily for the sake of the list itself. They are not part of the encyclopedia's categorization scheme.

The subcategories of this category are hidden categories. To add a category to this category, add ((Hidden category)), ((Maintenance category|hidden=yes)), or __HIDDENCAT__. A subcategory of a hidden category may or may not itself be hidden, depending on a template or a magic word in the subcategory or in a parent category.

This category is hidden on its member pages—unless the corresponding user preference (Appearance → Show hidden categories) is set.
These categories can be used to track, build and organize lists of pages needing "attention en masse" (for example, pages using deprecated syntax), or that may need to be edited at someone's earliest convenience.
These categories also serve to aggregate members of several lists or sub-categories into a larger, more efficient list (discriminated by classifications).

Description: This category contains hidden categories. Unlike normal categories, these are not displayed at the bottom of pages that are included in them, and are therefore hidden from readers.

Typically, categories should be hidden if they are concerned with the maintenance or administration of the Wikipedia project itself but include mainspace articles.

Previewing a page whilst editing it will list the hidden categories used at the bottom of the page. If you have created a user account, you can make hidden categories show up next to normal categories at the bottom of all pages by setting the user preference 'Show hidden categories'.