This is a Wiki. The concept is very simple; it's a big group of pages that anyone can edit at any time, with links and interconnections between each other. I didn't come up with it, however; you can find a much better example of a Wiki (complete with the philosophy behind it) here:
This Wiki is much smaller, and the rules are slightly different. However, the concept is the same. If you want to leave comments on any page, simply click the “Edit this page” link at the bottom of the page. This will open a page with the entire contents of the page you were looking at inside a text box. Add your comment to the bottom of the page, click “Save", and that's it.
Now, you might find it odd that you haven't even done so much as log in, and yet you have everybody's comments right there for editing. Well, there's no trick to it; there's no special algorithm that detects if it's really your comment or if you're trying to put words in someone else's mouth. The only thing that keeps a Wiki alive is the fact that the people using it don't want to be destructive.. after all, there's no challenge to it, so why bother?
That, and every change is recorded on the server, so if someone were to try and ruin things for everyone else, it would be easy to fix.. and then I would start implementing security measures on the Wiki. And that wouldn't be any fun. So let's keep it nice here, okay?
The Wiki uses slightly different formatting styles than you're probably used to. Read the Formatting Rules to learn how to write Wiki pages.
Since there is no built-in mechanism for distinguishing two people's comments, it's up to the users to make sure the text remains clear. Wiki Etiquette describes some useful styling guidelines.
Finally, you can try playing around with the Wiki in the Wiki Sandbox.
For some quick jumping-off points for thie Wiki, visit the Wiki Home.