Forums are sets of nodes and their comments. These are grouped together as belonging in one forum by assigning them the forum name, which is a taxonomy term. These forum names can be grouped in forum containers, which are other terms, of which they are children in a hierarchical vocabulary which is called "forums", and configured as only applicable to nodes of the "forum" type.
Links:
[1] http://li326-157.members.linode.com/about-realneo/realneo-capabilities
[2] http://li326-157.members.linode.com/blog/martha-eakin/posting-a-picture