The Forum addon adds a traditional topical forum to the channel on which it is installed.
The stream based interface of a "group channel" is a good tool for slow moving conversations, but things easily get "buried" in a larger group where a number of threads are created in a day. This makes it easy to miss important threads or forget about them as they move further and further down the page. The "Forum Addon" addresses this by creating a more traditional topical interface with each thread assigned to a specific topic and listed only by title. The table of contents is sorted in the order of most recently commented.
Standard "Group"/"Forum" channels require a separate channel for each group discussion topic in order to target group discussions to interested members. This increases the number of channels that need to be administered. It also increases the number of channels users need to track. The "Forum Addon" allows a single channel to host multiple topics that each has an independent, targetted distribution list. Optionally, users can be permitted to subscribe/unsubscribe themselves to specific topics.
Direct Messages (DM's) to standard "Group"/"Forum" channels are always sent to the group distribution list. This can cause difficulty or confusion in administerting a group/forum channel as there is no mechanism to distinguish "administrative" messages between a member and the channel owner from messages intended to be sent to the whole group. The Forum Addon only forwards messages that have an assigned topic to the distribution group. DM's that do not have a hashtag assigned to a topic only go to the channel owner. A topic can be assigned using hashtags from a forum member's stream or by using the Forum page which has a pull down of permitted topics.
Easy for forum participants to use
Like standard "Group"/"Forum" channels, posting can be done either on the channel forum page or from the user's own page. In the post form on the forum page, a dropdown is added for the poster to select a topic from the list of topics they are allowed to post to. Users may also post from their own timeline by sending a DIRECT MESSAGE to the forum channel and including an appropriate hashtag which is assigned to the topic. Permission to post can be controlled on a per-topic basis utilizing "privacy groups." Remote posters who do not have permission will have a comment sent back from the forum indicating their lack of permission.
Posts are forwarded using the same basic mechanism used by "Group"/"Forum" channels.
More info: Forum Addon Wiki Pages
See it in action on the DM42 Addons Channel