Although this is called the RNA Discussion Forum it is technically a discussion "board". A board can have a number of forums (main categories) and sub-forums (sub-categories). Forums and sub-forums can both have topics under them.
People often use different words to refer to forums, sub-forums and topics. Here are the most common examples:
forum = category = section
sub-forum = sub-category = sub-section
topic = post
reply = post