The Gaea Community uses a tag-based forum rather than categories. In essence, it is the same but with a modern twist on traditional forum categories. Tags can have 1 or 2 secondary tags - either from a parent tag's children (like Gallery has a child tag of Image) - or from one of the many secondary tags that can be used across any categories/tags, like Meta which can be used for anything regarding the forums themselves. For example, Bug + Meta means you're posting a bug about the Forums instead of Gaea itself.
To post in a specific tag, click the "Choose Tags" button in the post editor.
Or navigate to the tag/subtag you wish to post in and then start a discussion.
Inside a discussion
Once inside a discussion, you can choose how closely you wish to follow a discussion (or not at all!). Additionally, the discussion scrollbar lets you quickly see unread portions of a discussion, and go back and forth between the original and latest posts.
The Arrow button in the top left of the page always lets you see the forum list without having to navigate away.