If you are the moderator of a discussion forum, you can not only accept or reject posts sent to that forum, but also add users to the trusted users list, letting their posts bypass moderation.
On posting a message to a moderated discussion forum, the message is archived as moderation-pending until approved by a moderator. However, the messages posted by trusted users do not require the moderator's approval.
In an unmoderated discussion forum, messages are posted without being subjected to any screening. A user's ability to post is controlled only by the user's access permissions on the site.
On posting a message to an unmoderated discussion forum, a copy of the message is immediately distributed to all the subscribers of the discussion.
All the normal view and post rules are applicable to unmoderated discussions, except the rules applicable only to moderated discussions.
If you disable moderation on a moderated forum, the posts awaiting approval are automatically approved and sent to all the subscribers of the discussion forum.