A category is a software feature of MediaWiki. Categories provide automatic indexes that are useful as tables of contents.
To add a page to a category for example, type:
For example, to add an article to the category "Law", you would edit the article and add [[Category:Law]] into the page source somewhere. While exactly where doesn't matter but the convention is to put the categories at the end, after the article text. We encourage you to use Categories that have already been established. To see already established Categories see the Contents Page. For a complete list of categories, see Special:Categories.
A category page does not need to be created beforehand. Putting a category link in one of the articles will automatically create that category and display the page (or pages) that belongs to it. By looking at the very bottom of any article, you can tell what categories it belongs to, if any. Categories, if they exist, are always displayed there. Although a category has multiple articles in it, it will appear as a red link when a description of the category page is empty. To make the category links appear blue (like existing articles), click on the category link to go to the category page and edit it to add a short description.
When creating new categories, follow the conventions. For example, to create a category for a person, use this form Category:Last Name, First Name (yyyy-yyyy). After creating the category, add to the resulting page to have it appear in the complete People category Index. For US cities follow the format Category:State name-City, then add the Category:State name to the resulting page. For questions about how this functions, we suggest you look at how existing Categories have been set up.
Categories have sub-categories, meaning that you can put a category page inside of another category. Categories can be nested in multiple level in this fashion. For example "Social Sciences" category may contain a list of categories such as "Sociology", "History", etc. This is how the People category is set up, each category for an individual is sub categoried under People. It also allows so categories to be into several hierarchies for example the Category:Sodomy Law can go under both Category:Sodomy and Category:Law.
Linking to a Category Page
To link to a category page (without adding the page to the category), include a colon before the complete Category page name. For example, [[:Category:People]] renders as Category:People.
Singular or plural
It is not obvious whether a page like Amsterdam should be in category City (a description of a member of the category) or Cities (a description of the category as a set of pages). A convention for using one or the other is useful and should be codified. (Here is Wikipedia's convention). Sticking to the convention will help to avoid having both categories, with part of the pages in one, and part in the other.
Moving a category page
The only way to move a category page is to manually change all category tags that link to the category, and copy the editable part. There is no automatic way to move a category page in the way one moves an article page.