A forum is an online system which allows a group of users to communicate. Users can then post threads (essentially a starting point for a conversation) in a number of separate sub forums (each one usually covering a different topic).

A simple example forum structure would be as follows:

In this way, users can discuss numerous topics amongst themselves in a centralized community, in a more convenient way than other communication methods (e.g. email) can facilitate. Many websites have forums as a way for visitors to meet and discuss site content, and some sites are purely based around their forums. A number of administrators and/or moderators are also usually present on forum systems, making sure the forum runs smoothly and the users adhere to the rules specific to that forum.