In the context of online communities, a forum or message board is an online discussion site based around posting and responding to messages that tend to be longer than those sent via chat and which are archived, at least temporarily. In some cases, a moderator needs to approve the message before it becomes publicly visible.

Forums are one of the oldest forms of online social networking, originating in the bulletin boards of early dial-up Internet. Their format is hierarchical, with a single forum consisting of multiple subforums, each of which may contain a multitude of threads on various topics.

For example, a forum about snowboarding might contain subforums on different slopes, equipment, and events, with the subforums containing threads on recommendations for beginners, product comparisons, and meetups, respectively.