From Google's Groups page:
Usenet refers to the distributed online bulletin board system begun in 1979 at Duke University. Usenet users can post messages in newsgroups that can be read or contributed to by anyone with access to the Internet and special newsreader software. Over the years, the number of newsgroups has grown to the thousands, hosted all over the world and covering every conceivable topic.
Basically, "Usenet", or more broadly, "newsgroups", are an older form of communication. They're much like this forum system, except without all the bells and whistles - usually, you see only text. The main benefits are the enormous number of people who used to or still do use them, the huge selection of topics, and the loooong archives, that in some cases go back to the 1980s.
I'm guessing that Joe (or someone) got a plug-in or something that allows this forum to 'see' newsgroups from other sites, and display them here, just as an added resource.
If you go to groups.google.com and click on "about", you'll find a longer, better explanation.
Hope this helps