When you create a topic (aka. thread), you will see a dialogue (a white text box).
![Image](https://i.imgur.com/XdeQR7U.png)
You can enter a tag (e.g., philosophy, entertainment, game, whatever) and when you hit enter, the tag will be added to the topic.
Note that spaces will be replaced with '-'.
You can do the same for existing topics by editing the first post of your topic.
At the bottom of the main page you will find a tag cloud with the most common tags.
Anyways, it's a bit of an experiment. Most modern content management systems use tags rather than some pre-set hierarchical structure. I hope this can give a bit of structure to different themes without needing lots of forums and sub-forums.
Might also tag some of y'all's existing topics. Would that be rude?