The Usenet is something like a very, very, very big bulletin board. In "former times" it has been one the most used services of the internet, but today most user don't know it :-/
There are newsservers with thousands and thousands of newsgroup, with are sorted hierarchically and where you can talk about every topic you every wanted. But there are also so called binary groups, where you can download or upload binaries (pictures, programmes, songs, movies, ...).
Google provides a webinterface for the Usenet (and call it "groups") at
http://groups.google.com/
But normally you access the Usenet via a newsreader - most mailclients also can access the Usenet. And it's much easier to follow discussions in the Usenet than on webboards like vBulletin. You can ignore subthreads, write rules (e.g. if sender is "john doe" give 50 points. if points >= 50 set color to yellow) and so on.
For more information see
http://en.wikipedia.org/wiki/Usenet
A very powerful newsreader
http://pan.rebelbase.com