In correctness, it should be forums as each main area in a community site such as DNForum.com has a number of forums under it. As you notice on the DNForum.com home page there are the main topics: Domain News, Beginners Guides and Legal Stuff!, Content Development, Exclusive Forums (note the "s"), etc. Under each such topic are the forums (as denoted by the header below each title "forum"), some forums have sub-forums and so forth.
So a forum is only a section of a community discussion site. However, popular usage has reduced forums to just forum.