Porshuh944turbo has a pretty thorough explanation of the binary transfer (file sharing, that is) scene on Usenet, but there was one slightly confusing error:
Originally posted by Porshuh944turbo
Servers generally describe what you should be posting and downloading from them (like alt.binaries.mac.apps) has a lot of cool apps for mac.
Each of those groups, like alt.binaries.mac.apps, is called a newsgroup, not a server. The entire set of newsgroups you have access to are all on the same server.
If you're interested, the Usenet's structure is something like this: There are a whole bunch of Usenet servers on the Internet. Each server is linked (has a feed) to at least one other Usenet server.
Periodically, a server will connect to whatever other server(s) it is associated with and compare the list of posts in each newsgroup that is on that server. If server A has a new post that server B doesn't, it sends that post to server B. If server B has a post that server A doesn't, A grabbs that post from B.
This chain reaction is carried out between all the various Usenet servers on the Internet, and so eventually a post that starts on a single server (whichever one the poster uses) will be found on EVERY server--usually called propagating.
How long any given post stays on any given server is entirely up to that server's administrator, usually based on how much storage they can afford. That's called retention time, and the longer the better.
Groups where massive multipart files (gigabytes a day of data) are traded rarely have a retention time of more than a few days other than on expensive commercial servers. Text-only (discussion) groups--basically the predecessor to forums like this one--can easily have retention times in the weeks or months, since traffic is so much lower.
Sometimes posts don't make it to a particular server--say, the server they originated on forgot to transfer them, so they never propagated at all, or just some particular flakey server didn't grab it along the way, causing some downstream servers to miss it. Big expensive Usenet providers usually have multiple newsfeeds, making it less likely that the'll miss a message, but it can still happen, particularly when someone posts using a flakey small-time server.
Finally, not all Usenet providers carry every newsgroup. There are over 100,000 groups at this point (many of them dead or jokes to begin with, but tens of thousands are still active), but a local ISP probably won't carry more than a few thousand of the more popular text groups. Big dedicated providers try to carry them all, and come darn near succeeding.
Last thought: An interesting difference between P2P filesharing and Usenet binary groups (don't forget that it's still a huge discussion community as well) is that while P2P is on-demand but also highly encourages (or forces) participation, Usenet is a one-to-many distribution medium. When someone feels the need to share something, everyone benefits equally, and the cost of distribution is spread out among paid Usenet providers and ISPs that include Usenet service. Personally, although it can make finding exactly the file you want exactly when you want it harder, I prefer that system.
In my case I dabble a bit in the discussion groups, but I mainly use Usenet to satisfy my MAME ROM fix.
Cox@Home actually has a pretty good newsfeed included (close to a week retention even on some very high traffic binary groups), but I also use Giganews when I can afford to pay for the good stuff--great retention and fast servers.
Oh, and if you decide to do some binary downloading, get yourself Thoth--it's a great app.