http://en.wikipedia.org/wiki/usenet
"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 (Think of it as a spider web. There are multiple ways to get to any point in the web, so if any of those ways is blocked for any reason, there is always another way to get there, allowing the information to reach its destination safely. In this manner, one can easily see that it would be hard indeed to prevent the spread of information across Usenet.) Individual users may read messages from and post messages to a local server operated by their ISP, university or employer. The servers then exchange the messages between one another, making the messages available to readers beyond the original server.
Removal of copyrighted content from the entire usenet network is a nearly impossible task, due to the rapid propagation between servers and the retention done by each server. Petitioning a usenet provider for removal only removes it from that one server's retention cache, but not any others. It is possible for a special post cancellation message to be distributed to remove it from all servers, but many providers ignore cancel messages by standard policy, because they can be easily falsified and submitted by anyone. For a takedown petition to be most effective across the whole network, it would have to be issued to the origin server to which the content has been posted, but has not yet been propagated to other servers. Removal of the content at this early stage would prevent further propagation, but with modern high speed links, content can be propagated as fast as it arrives, allowing no time for content review and takedown issuance by copyright holders.
Establishing the identity of the person posting illegal content is equally difficult due to the trust-based design of the network. Like SMTP email, servers generally assume the header and origin information in a post is true and accurate. However, as in SMTP email, Usenet post headers are easily falsified so as to obscure the true identity and location of the message source. In this manner, Usenet is significantly different from modern P2P services; most P2P users distributing content are typically immediately identifiable to all other users by their network address, but the origin information for a usenet posting can be completely obscured and unobtainable once it has propagated past the origin server.
Also unlike modern P2P services, the identity of the downloaders is hidden from view. On P2P services a downloader is identifiable to all others by their network address. On Usenet, the downloader connects directly to a server, and only the server knows the address of who is connecting to it."
Originally Posted by fatcat69
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