What's the very last thing you'll find at the bottom of a Wikipedia page? Categories. Here you'll find a few (or many) links named for a group of topics, with links to similar pages, intended to help the reader navigate the topic and discover related ones.

For example, the Super Tuesday entry lists among its categories: United States presidential primaries and, surprising or not, Tuesday. Click on one, and you'll be taken to a page listing other relevant articles (i.e. Mardi Gras).

Simple as it may seem, categories can be contentious when editors disagree about how best to group information. (See, for example, the "American women novelists" controversy of 2013.)

If you're curious to find some categories for perusal, the Categories portal is the best place to start. Finally, for fun: Wikipedia, ever self-referential, of course has a category of Wikipedia categories.