If you ask this question on the Web, you'll find plenty of answers , not one of which will actually explain what you want to know.
Truth is, it's a lot simpler than that.
People will deny that it's anything like a mailing list or a bulletin board, but from the user's point of view, it's very like them. It's as simple as that. On a high-traffic newsgroup, your message may well be seen by tens of thousands of readers worldwide. One of my favourite hobbies is Usenet. If you know what Usenet is, you can usually find me on alt.fan.pratchett , posting as Miq.