What is the forum, and how do I use it?
Explaining forum functionality in the context of the site is a bit of a big task, but the basic idea of a forum is a collection of conversations and discussions participated in by users on topics relevant to their interests. Forums/threads are a subfunctionality of groups, and can be accessed by clicking on the 'forum' button within a group.
Replying to threads works just like comment replying, bbcode and all. If you have further questions about how forums work in general, more information can be found here