The original message boards or "forums" of the InterNet, made up of millions of NewsGroups and transported using the NNTP protocol. You use a NewsReader to browse it and post messages.
There is an extensive archive, originally by DejaNews (who no longer exist) and now known as GoogleGroups. A large "dark half" of UseNet is comprised of (often illegal) binary traffic (the hierarchy is mainly under alt.binaries, and let's just say erotica features highly in the lists). Messages containing file( fragment)s are MIME encoded and most news servers don't carry them.
UseNet was one of the first things on the InterNet in the early 1980s, full of VAX and UNIX questions. Linux is a newer thing and tends to be best supported in MailingLists - MicrosoftWindows queries on the other hand are best answered in MicrosoftCorporation's "private" UseNet NewsGroup hierarchy, 9 times out of 10.