Categories are a way of grouping together pages with some related theme or element. It indexes them in an easy to use way.
Sounds rather dry, doesn't it?
But Categories are actually an incredible way of collecting wiki pages together in ways that help people connect, to find useful information, or to bring out narratives of the history of the East Kingdom.
Let's dig into Categories a bit. All the current categories with at least one page in them can be found at Special:Categories, and it's a really long list. That's because anyone can create a new category, as well as use any of the existing categories. All you have to do is put [[Category:Category name]] on a page (throwing it at the bottom will keep it out of your way when editing other things) and the page will be added to that category. That's really all there is to it. And if you want to link to a Category (say... Category:English_Brewing), all you have to do is put a colon (:) before the word Category. Like so: [[:Category:Category name]]
So, what sorts of categories are in the wiki? There are categories for personas from certain time periods (Category:14th Century Personas) or from certain cultures or period geographies (Category:Irish Personas). There are categories for SCA regional groups (Category:Barony of An Dubhaigeainn) and activities (Category:Equestrian). There are categories such as Category:Poems and Songs and Category:Regalia indexing things you can find in the wiki. There's a category for all the populace pages that haven't yet linked to an OPID entry (Category:Needs OPID). And there is even a category honoring absent friends (Category:Memoriam). Categories can help us identify key information about a page, to help everyone to discover new related pages, or even to help tell the story of the East (Category:History).
Further, every Category page is itself a page that can be edited. So a Category can be placed within another Category, by adding the right link. This means categories can be nested into useful hierarchies. See Category:Brewing as a category that has subcategories, indexed pages, even an image associated. Categories are a powerful way for self-organization to happen. So add the appropriate categories to the pages you edit!