I'll try to explain. (I just use usenet without understanding it)
Usenet is like a public forum. It consists of thousands of newsgroups (is that the right term?) to which people could post messages and start discussion threads. If you want to compare it to AT forums, each newsgroup is comparable to a AT forum category. You can post messages to each newsgroup, and if someone replies to your message, it becomes a thread. The newsgroups are sometimes moderated, mostly not. The unmoderated ones are mostly spam! Other than the moderators, I get the impression that Usenet is unregulated--there are all kinds of MP3 music to steal!
Many people find Usenet to be a great source of MP3s and pr0n. :evil: My understanding is that Outlook Express is not very good for downloading Usenet attachments (aka "binaries"). I use a shareware program called "NewsShark."
Just be real careful.