Dungeons and Dragons Wiki:Category Structure
Categories are used on the wiki in a bush structure, rather than a tree structure. A category describes a basic set that the page is in, rather than any complex details. You will not see categories "Publications from 2003" for example, but you would see "Publication". This is because we have special tags to deal with this, called semantic-mediawiki, to give the details of when and which. Because of this, a category should answer the question "What is this?" It is a user-created mundane weapon from 3.5e, a type of equipment. That is what categories answer.
So, what categories should you use for your pages? Well, if it is a homebrew article, the category User is required, and the category for the version (3.5e or 4e) should be supplied. The name in the Identifier should also be supplied. For example, if its an equipment page, then you would want the category Equipment.
Furthermore, for certain types of articles, mainly classes and equipment, a few more categories are needed. For equipment, you should have a category for whether it is magic or mundane and what type of equipment it is. The preload shows the acceptable categories. For classes, you need a category to show what type it is.
All other data is described via semantic mediawiki. You don't have to worry about it so much, but if you want to know, ask on IRC. Just don't add categories randomly. The only exception is for sourcebooks. If you have content spread through the wiki and transclude it into your sourcebook, you can have a category. One example is Chocobo used for the Chocobo project.
You may not have a user category. In the Semantic Search, search for [[Author::<authorname>]] (without angle brackets) if you want to find all pages by a specific author.