From Sega Retro
You can place an article in a category by adding the string [[Category:''Example'']] to the article's text (where Example is the name of the category). Though there are the main categories - Games, Characters, Items, Hardware, Developers, Sites, Sceners, Hacks and Hacking Information, a full list of categories may be found at Special:Categories.
The category feature has several functions:
All articles in a given category are sorted alphabetically. If you want an article to be sorted by something other than its name, you should use the following link form in the article's text: [[Category:''Example''|''Sort key'']] (where Example is the category name, and Sort key is the value to sort the list by).
For example, by default, the article Yuji Naka will be listed under his first name, "Y", because it is the first letter of the article's name. You could instead force the article to be listed by the character's last name ("Naka, Yuji") by using the string [[Category:Developers|Naka, Yuji]].
Linking to a category
In order to link to a category without the current article being added to that category, place a colon before the word "Category" inside the link.
Example: [[:Category:Games]] produces Category:Games
Category pages may be added to other categories and are then recognized as sub-categories. For example, you could edit Category:Mega Drive hardware and add the link [[Category:Hardware]]. The Mega Drive Hardware category would then be a sub-category of the Hardware category.