The usenet is all three.
When I was a lad, the usenet was used for posting to boards - like an old fashion forum.
To use the usenet you need a program, called a newsreader, e.g. newsleecher. You need a subscription to a usenet provider (usenext or gignews for example), and the usenet a collect of very large servers that you can download at very high speed. There are boards where you can post on any topic.
The usenet these days are used for uploading and dowloading copyrighted material - movies, software, music etc.
If you search for free usenet trial, you can normaly get an amount of GB download free.
If you just want to visit "groups" try google groups at click here