This sites uses two ways to classify pages, Categories and Tags.
Categories are groups of related pages. A page can only belong to one category. For example, all game pages belong to the "game" category. All the designer pages belong to the "designer" category.
To create a page that belongs to a category just add the name of the category before the name of the page, and separate them with a ":" symbol. For example: http://decktet.wikidot.com/game:ace-trump
If the page doesn't exist, you'll be given the option to create it.
Anyway, there's a short cut to create new game pages. The form on the sidebar automatically adds the page to the game category.
You can make lists of pages belonging to a category. For example:
[[module ListPages category="game" order="titleAsc"]]
This would make a list of all the games (and you can see the result at the All the Games page.
Tags are labels that can be applied to existing pages. Just click on "Tags" on the bottom bar of a page and you can add as many tags as you want to a page. Tags can't contain spaces, so you'll see many tags like: work-in-progress, etc…
On the sidebar there's a tag cloud. Clicking on a tag name will take you to a list of pages that have that tag. But you can also make lists of pages that have (or don't have) certain tags. For example:
[[module ListPages category="game" order="titleAsc" tag="tableaux-building"]]
This would list all the games that have the "tableaux-building" tag, so it's a great way to further classify games using different criteria like mechanics, themes, number of players, etc…
Feel free to add your own tags and create your own lists, but be sure to check the List Pages Module Help to see how to format the lists you can automatically create.