This was Apple Discussions before it became Apple Support Communities.
Apple Discussions was a conglomerate of various communities, whose subset were forums, and those were grouped into various threads of several posts a piece.
Under the Apple Support Communities the arrangement got murkier.
Now it is Community (think the Notebooks), followed by subCommunities (MacBook Pro), followed by user selectable categories (Using MacBook Pro), which may not be selected at all when posting. As such you are going to have some non-categorized threads which are invisible if you select a Category, while other threads clearly are part of a category. And of course threads constitute a multitude of posts. And each thread can be viewed in a tree or a flat mode
If you start at http://discussions.apple.com/ you'll be able to see the hierarchy I talking about. Each thread or community can be viewed threaded, or flat, or RSS depending on your user preferences.
Yes, this can be kind of confusing, but the that's how it is set up. At least now, you can post a new topic while viewing a thread, where before you couldn't. It still becomes difficult when posting "ask a question" form a knowledgebase link, or navigating from another internet search engine, or the Support pages of Apple to determine where your post might end up. Worse yet, sometimes when you quit your web browser you are left in the same thread you were before, and somtimes you are totally logged out having to begin again anew depending on whether you use your own machine, or a machine whose security is set differently, or an iOS device. I try to start at the first link whenever I log in, as long as I don't see something in my e-mail I want to respond to. Others may have other ways of navigating it. This another reason I encourage people to start their own thread. Because if they just post at the end of a thread, I may lose interest in a thread, and not follow it anymore, and just look for threads with no replies.