Put simply, Usenet is the Internet's oldest message board. Containing tens of thousands of groups catering to every imaginable topic, everybody sees what everybody else posts. The Usenet network is unique: when a user posts a message on a Usenet server, that message is transferred from news server to news server, all around the world, until every Usenet server (hopefully) contains a copy of that message.

Thus, every Usenet server, while being independent, is theoretically a mirror of all the others!

Originally, Usenet was the only way to group-communicate globally -- Usenet archives date back to 1981 (!). Even today, Usenet is still the place for awesome global conversation -- everything from good cheesecake recipes to politics to what's the best Mac FTP client. (Which shouldn't be much of a debate. Ahem.)

Lately, Usenet also hosts a large number of "binary groups". These groups contain files of every type -- movies, music, photos, etc. Unison will automatically piece these files together and make them easy to get!

Of course, like the Internet itself, Usenet is a vast, unmonitored wilderness -- nobody can control the content posted to the servers, so caution is advised as you navigate. But, with a little curiosity and investigation, you may find Usenet to be a surprisingly rich new Internet resource -- one that's actually been there all along!