Usenet is a "network" of newsgroup servers (often run by ISPs) working together. Once connected to Usenet one can find everything from logical conversation to porn. One bad thing about Usenet is that it is often used to distribute warez, or illegal digital content like mp3s, Screeners, or cracked video games. In most cases Usenet is the first step in the life of warez. Unfortunately this has caused many newsgroup servers to block access to known warez newsgroups.
Usenet providers normally charge money for access but many ISPs have newsgroup servers running that their customers can access for free. So if you would like to check out Usenet first call your ISP and find out if they have a newsgroup server running.