They differ from chat rooms in that messages are often longer than one line of text, and are at least temporarily archived.
A user of the forum can automatically be promoted to a more privileged user group based on criteria set by the administrator.
A forum consists of a tree-like directory structure. A forum can be divided into categories for the relevant discussions.
Under the categories are sub-forums and these sub-forums can further have more sub-forums.
If messages are not related to one another at all a Non-Threaded format is best.
If a user has a message topic and multiple replies to that message topic a semi-threaded format is best.