Hidden categories are those that have been tagged with the magic word: __HIDDENCAT__.
Hidden categories do not appear listed on articles. If you are wondering why the category you have added to an article has not appeared, check to see if the category is hidden. Previewing a page will list hidden categories at the bottom of the page, below the list of templates used.
You can make hidden categories visible by going to your preferences, under the "Appearances" tab.