Usenet is a distributed discussion system. It consists of newsgroups each of which are devoted to different subjects.
Usenet is a globally distributed system. Newsgroup postings made on one server are often distributed to other newsgroup servers around the world. Any one newsgroup server will typically "carry" (or provide) a selection of newsgroups. Generally newsgroup access is via a news-server provided by your ISP. Occasionally it may be necessary to make use of other news-servers - for example where your ISP's newsgroup server does not carry a group that you are interested or where your ISP does not provide access to newsgroups.
Usenet is also a public discussion forum, anyone can participate and the discussions are available for anyone to view.