Here’s the basic idea: Topic pages declare values for facets. These are basically just categorized tags. So my Message Board tutorial has some simple markup that says it’s written in C and it’s teaching you WebKit. You then have a page that collects all the topics and provides selectors to narrow what you’re looking at.
So as I browse the demos, I can say that I’m only interested in C. And perhaps I’m looking at GTK+ and Clutter to see which one is a better fit for a project I’m starting. I can uncheck everything else. Then I’m looking at only demos in C that teach either GTK+ or Clutter. Screencast:
(Aside: When I try to upload an ogv file to WordPress, it tells me the file type doesn’t meet security guidelines. If I rename it to ogg, I can upload it, but WordPress inserts it as audio. I have to write the HTML for the video by hand. Could somebody please make it easier to use Ogg Theora on blogs.gnome.org?)