Releases from the stable branch will get bug fixes and security fixes only, all new features are introduced in the development branch. This text is similar to the contents of the online help page, but contains additional admin info. It's however not always as current as the online help. The forum is the whole installation, containing multiple categories and boards. A board contains all the threads on a topic.
Boards can be private, so that they are only visible to admins and members.
Boards can be semi-anonymous, which means that poster names will be shown to admins only. People will still need an account for posting, however. Both options are usually used for announcement boards. Another option for boards is approval moderation. If this option is activated, new posts will be invisible to normal users until an admin approves them.