A forum, or message board, is an online discussion area where people can hold conversations in the form of posted messages.
Forums have a specific set of jargon associated with them; e.g. a single conversation is called a "thread" or discussion.
A discussion forum is hierarchical or tree-like in structure: a forum can contain a number of sections, each of which may have several categories. Within a forum's category, each new discussion started is called a thread, and can be replied to by as many people as so wish.