Essential is for me the post. Without the post, a forum would be pointless. You can argue about organizing posts into threads and threads into forums. You could avoid having forums by tagging threads. Or you can have the same posts within different "threads" or whatever you call it then.
From what I get, most people want to store text information and image information, that's it. It is important how you organize them: as blogs, within cms types, articles, wiki.