Wikipedia has index pages called Categories, in which links to articles are provided based on category tags included in each article.
To add an article to a particular category, add [[Category:Category name]] to the article's text (preferably at the bottom). The assigned category should now show up at the bottom of the article, and a link to the page should show up on the indicated category page in Wikipedia's category system. If the category shows up as a blue link it is an active category; if the link shows up red then the category has yet to be created. Click on the red link to create the category page.
Done
