Wikipedia's four million articles are an impressive achievement, but the content of the encyclopedia isn't in the entries alone: The structure is content too.
That Wikipedia exists at all is remarkable. In little more than a decade, millions of people have collaborated to build what is perhaps the world's largest group project, a compendium of some articles on some four million different topics.
But I've realized that as impressive as those four million article pages are, they don't capture the entirety of why Wikipedia is useful. Wikipedia's functionality lies not just in its encyclopedia contents, but in the structure of how those pages connect together: the often-used, little-remarked-upon category pages. For too long I have taken these category pages for granted, but no longer. This is an appreciation for categories -- the way they help us find what we're looking for, the information they subtly embody, and the Wikipedians who build them.
Category pages were first enabled in May of 2004; they have since exploded (a list available here is so big I don't recommend clicking unless your computer is very capable).
It's not an exaggeration to say there are categories for pretty much everything: endangered plants (quite long, sadly), every single decade of film (and, even more narrowly, the same for more specific genres such as drama or pornography), concepts in cognition, and, one of my personal favorites, Lists of English words of foreign origin. If you are looking for a list of researchers focused on a certain area, Wikipedia will provide. If you want to know which movies a particular musical composition has appeared in, Wikipedia has that too. For this kind of information -- the grouping of unrelated items connected by a single thread -- there is really no other easy way to find this information online.
If you've never quite paid attention to categories before, or if you've landed on them but don't really know how, you can find them at the bottom of any Wikipedia article in a little nondescript box. Here, for example, is the category box for Canadian science-fiction writer Margaret Atwood:
It's not flashy and it doesn't -- at first -- seem imminently useful. But if you're a fan of Atwood's, the links to "Booker Prize winners" or "Women science fiction and fantasy writers" will bring you to more genres, books, and authors than even the most detailed biography. Suddenly, you aren't reading just about Atwood, but about where she sits in the world of writers and writing; the structure of Wikipedia becomes its own sort of content.
In addition to categories, one certain kind of Wikipedia article page fulfills a similar function: list pages. List pages preceded categories -- since they required no infrastructure beyond that of a regular Wikipedia page, whereas categories rely on automated functions that had to be built into the system -- and there is some discussion on Wikipedia about whether they should even continue to exist, in part because there is a lot of repetition (compare, for example Category: Cognitive Biases with List of Cognitive Biases). The consensus seems to be that the duplication doesn't do any real harm, and that there are a few advantages to maintaining both systems. (For a collection of some top-notch Wikipedia lists, you can do no better than this Tumblr, List of Lists on Wikipedia.)
In either case -- lists or categories -- the structure is more powerful than just links: A linked entry could take you from a writer to the Wikipedia page of where that person was born, but what then? Categories and lists zoom you out further, not just to a birthplace, but to all the people who share it. This framework gives Wikipedia a functionality kind of like a brain, sorting and connecting people, places, and things that are in some ways associated. It is what the cognition of the mind looks like in the code of a giant website, letting you click much like you would think.