Articles may be classified according to categories. Proper classification of articles can make searching the wiki much easier.
An article can be in any number of categories.
Use the page Special:Categories to see a list of categories. There is a link to it in the sidebar on the left side of every page. <-- OR THERE SHOULD BE! -->
Follow a category link in the list of categories to see all the articles (pages) in that category, organized alphabetically.
The categories for an article are listed at the bottom of the page. Each is a link to a list of all pages in that category.
To put an article in a category you simply need to add a category link to the article. A category link looks like:
How you pick the categories is up to you. Be careful to choose reasonable categories that make sense for the subject matter in the wiki. There's no point in having each article in it's own category, or in having all articles in the same category. Use the category mechanism to help divide up the wiki for easier searching.