A category is a WikiName that uses reverse linking: if you click on the title of a category page, you'll get a list of pages belonging to that category. To get a list of all categories, click above on the CategoryCategory title.
Additionally, you can add lists of all pages which are members of that particular category to the category page using macros like this:
Here is a list of all categories known to this wiki:
Here is a list of all pages containing the CategoryCategory wiki tag:
To be consistent with the C2 category scheme, all categories start with the word "Category".