This wiki has always had the feature that clicking on a page title when viewing the page produces a list of all the other pages that link to that page. Other wikis have this feature too. In September 2001, WHD added categories to the wiki, using only this feature, i.e. he added no code to the wiki software. See the comp.lang.tcl post at https://groups.google.com/forum/#!original/comp.lang.tcl/4cIvZt2VA44/TzhWkf3AimgJ .
Categories are very simple. Suppose you want to provide an index of all pages on the Wiki relating to the WiKit software. To each such page, add the following link: [Category WiKit]. Then, create a [Category WiKit] page that defines the purpose of the category. It need not have much text on it; its primary value is that if you click on its title, the search features built into the Wiki will give you a list of all pages that link to [Category WiKit] ...
and, finally,
Category Category
That last is the category to which all Category pages belong, so that you can go to Category Category, click on its title, and get a list of all Category pages.
That's it. (except that you can't click on the page title any more, you have to choose References from the Page menu.)
By convention, the links to a page's category or categories go at the end of the page, and there is now the <<categories>> macro to format them.
<<categories>> Topic | Other Topic
However that macro has no function other than formatting, and the standard wiki functionality doesn't care where in the page they are.
Another convention, obviously dating back to WHD's original, is that category pages have names starting with the word "Category" so you can see what they are. However, any page that has a link to [Category Category] behaves like a category page, so it might be considered to be one, and the <<categories>> macro allows that by formatting the above example as if it were specified as
!!!!!!
| [Category Topic] | [Category Other Topic] |
!!!!!!
prepending the word "Category" unless there is only a page (e.g. "Topic") without it and no page (e.g "Category Topic") with it.
The whole category thing is automatic, except that the original version of this page said:
"An attempt is being made to produce some sort of self-categorizing of the pages on this web site. To do this requires one to enter at the bottom of pages one or more links to one of these tags:"
followed by a very short list of categories. Hence the rather longer and necessarily always incomplete list below.
The last quote also shows that categories are just tags with no hierarchy or any other structure.
PYK 2015-11-08: In that case, "categories" is an unfortunate name for this system, as "keywords" would have been more fitting. It goes to show that the idea was half-baked from the beginning, and that initial descriptions of it shouldn't be taken too dogmatically.
EMJ 2015-11-08: The name was already used by other wikis for the same sort of concept, so why not? As for being half-baked, no, it seems to have been more of "hey, we can do what they do with our existing functionality, we just have to use it in a particular way". And at the time (yes, I was here then!) it looked like a clever and wonderful idea. Also, there is no reason why "A class or division of people or things regarded as having particular shared characteristics" ( http://www.oxforddictionaries.com/ ) has to be part of a hierarchy (though of course it can). For an explanation of why I and others have a very deep suspicion of any attempt to turn even part of the Wiki into to a hierarchy, ask someone about the unperson episode. Hierarchies are useful, but they are not essential, and can even be used to hide things. They are also, by the way, contrary to the spirit of the internet.
Major Categories
In addition to the list of categories automatically generated by clicking on the title of the Category Category page, the following is a manually-curated list of major categories (where, for the time being, "major" means "in this list"):