Usenet (USEr NETwork) is a global, decentralized, distributed Internet discussion system that evolved from a general purpose UUCP architecture of the same name. It was conceived by Duke University graduate students Tom Truscott and Jim Ellis in 1979.

Users read and post e-mail-like messages (called "articles" or "posts") to one or more of a number of categories, called newsgroups. There are well over 100,000 'newsgroups' so finding a topic of interest is not hard at all!

Usenet has no owner or central server. It is arguably the most expressive and unmoderated form of communication on the Internet.

Usenet is distributed among a large, constantly changing conglomeration of servers which store and forward messages to one another. These servers are loosely connected in a variable mesh. Individual users usually read from and post messages to a local server operated by their ISP, university, employer, or some other local organization. Then, the servers exchange the messages between one another. Usenet has been described as a system of online collaboration and interaction similar to today's Web 2.0. It has also been pointed out that its decentralized structure makes it more democratic than Web 2.0.

So, to put it simply...

Usenet is the largest online discussion community with over 100,000 groups to choose from.

Discussions are threaded (as opposed to linear post after post style forums), so they often drift off into various directions meaning that you can pick and choose which parts of a discussion you choose to follow or participate in. It's like a mind map of ideas.

The discussion groups are generally not moderated and, given the rapid way in which your posts are indexed on thousands of other newsgroup servers, they cannot be edited, deleted or modified.