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A Usenet newsgroup is a repository usually within the Usenet system, for messages posted from many users in different locations. The term may be confusing to some, because it is in fact a discussion group. Newsgroups are technically distinct from, but functionally similar to, discussion forums on the World Wide Web. Newsreader software is used to read newsgroups.
Types of newsgroups 
Newsgroups generally come in either of two types, binary or text. There is no technical difference between the two, but the naming differentiation allows users and servers with limited facilities to minimize network bandwidth usage. Generally, Usenet conventions and rules are enacted with the primary intention of minimizing the overall amount of network traffic and resource usage. Typically, the newsgroup is focused on a particular topic of interest. Some newsgroups allow the posting of messages on a wide variety of themes, regarding anything a member chooses to discuss as on-topic, while others keep more strictly to their particular subject, frowning on off-topic postings. The news admin (the administrator of a news server) decides how long articles are kept on his server before being expired (deleted). Different servers will have different retention times for the same newsgroup; some may keep articles for as little as one or two weeks, others may hold them for many months. Some admins keep articles in local or technical newsgroups around longer than articles in other newsgroups.
Back when the early community was the pioneering computer society, the common habit seen with many articles was a notice at the end disclosed if the author was free of, or had a conflict of interest, or had any financial motive, or axe to grind, in posting about any product or issue. This is seen much less now, and the reader must read skeptically, just like in society, besides all the privacy or phishing issues.
There are currently well over 110,000 Usenet newsgroups, but only 20,000 or so of those are active. Newsgroups vary in popularity, with some newsgroups only getting a few posts a month while others get several hundred (and in a few cases a couple of thousand) messages a day.
Non-Usenet newsgroups are possible and do occur, as private individuals or organizations set up their own NNTP servers. Examples include the newsgroups Microsoft runs to allow peer-to-peer support of their products and those at news://news.grc.com.
Binary newsgroups 
While newsgroups were not created with the intention of distributing binary files, they have proven to be quite effective for this. Because of the way they work, a file uploaded once will be spread and can then be downloaded by an unlimited number of users. More useful is that every user is drawing on the bandwidth of his or her own news server. This means that unlike P2P technology, the user's download speed is under his or her own control, as opposed to under the willingness of other people to share files. In fact, this is another benefit of newsgroups: it is usually not expected that users share. If every user makes uploads then the servers would be flooded; thus it is acceptable and often encouraged for users to just leech.
There were originally a number of obstacles to the transmission of binary files over Usenet. First, Usenet was designed with the transmission of text in mind. Consequently, for a long period of time, it was impossible to send binary data as it was. So, a workaround, Uuencode (and later on Base64 and yEnc), was developed which mapped the binary data from the files to be transmitted (e.g. sound or video files) to text characters which would survive transmission over Usenet. At the receiver's end, the data needed to be decoded by the user's news client. Additionally, there was a limit on the size of individual posts such that large files could not be sent as single posts. To get around this, Newsreaders were developed which were able to split long files into several posts. Intelligent newsreaders at the other end could then automatically group such split files into single files, allowing the user to easily retrieve the file. These advances have meant that Usenet is used to send and receive many terabytes of files per day.
There are two main issues that pose problems for transmitting binary files over newsgroups. The first is completion rates and the other is retention rates. The business of premium news servers is generated primarily on their ability to offer superior completion and retention rates, as well as their ability to offer very fast connections to users. Completion rates are significant when users wish to download large files that are split into pieces; if any one piece is missing, it is impossible to successfully download and reassemble the desired file. To work around the problem, a redundancy scheme known as PAR is commonly used.
A number of websites exist for the purpose of keeping an index of the files posted to binary newsgroups.
In part because of such long retention times, as well as growing Internet upload speeds, Usenet is also used by individual users to store backup data in a practice called Usenet backup, or uBackup. While commercial providers offer more easy to use online backup services, storing data on Usenet is free of charge (although access to Usenet itself may not be). The method requires the user to manually select, prepare and upload the data. Because anyone can potentially download the backup files, the data is typically encrypted. After the files are uploaded, the uploader does not have any control over them; the files are automatically copied to all Usenet providers, so there will be multiple copies of it spread over different geographical locations around the world.
Moderated newsgroups 
A moderated newsgroup has one or more individuals who must approve articles before they are posted at large. A separate address is used for the submission of posts and the moderators then propagate posts which are approved for the readership. The first moderated newsgroups appeared in 1984 under mod.* according to RFC 2235, "Hobbes' Internet Timeline".
How newsgroups work 
Newsgroup servers are hosted by various organizations and institutions. Most Internet service providers host their own news servers, or rent access to one, for their subscribers. There are also a number of companies who sell access to premium news servers.
Every host of a news server maintains agreements with other news servers to regularly synchronize. In this way news servers form a network. When a user posts to one news server, the message is stored locally. That server then shares the message with the servers that are connected to it if both carry the newsgroup, and from those servers to servers that they are connected to, and so on. For newsgroups that are not widely carried, sometimes a carrier group is used for crossposting to aid distribution. This is typically only useful for groups that have been removed or newer alt.* groups. Crossposts between hierarchies, outside of the Big 8 and alt.* hierarchies, are failure prone.
Newsgroups are often arranged into hierarchies, theoretically making it simpler to find related groups. The term top-level hierarchy refers to the hierarchy defined by the prefix before the first dot.
The most commonly known hierarchies are the Usenet hierarchies. So for instance newsgroup rec.arts.sf.starwars.games would be in the rec.* top-level Usenet hierarchy, where the asterisk (*) is defined as a wildcard character. There were seven original major hierarchies of Usenet newsgroups, known as the "Big 7":
- comp.* — Discussion of computer-related topics
- news.* — Discussion of Usenet itself
- sci.* — Discussion of scientific subjects
- rec.* — Discussion of recreational activities (e.g. games and hobbies)
- soc.* — Socialising and discussion of social issues.
- talk. * — Discussion of contentious issues such as religion and politics.
- misc.* — Miscellaneous discussion—anything which does not fit in the other hierarchies.
These were all created in the Great Renaming of 1986–1987, before which all of these newsgroups were in the net.* hierarchy. At that time there was a great controversy over what newsgroups should be allowed. Among those that the Usenet cabal (who effectively ran the Big 7 at the time) did not allow were those concerning recipes, drugs, and sex.
This situation resulted in the creation of an alt.* (short for "alternative") Usenet hierarchy, under which these groups would be allowed. Over time, the laxness of rules on newsgroup creation in alt.* compared to the Big 7 meant that many new topics could, given time, gain enough popularity to get a Big 7 newsgroup. There was a rapid growth of alt.* as a result, and the trend continues to this day. Because of the anarchistic nature with which the groups sprang up, some jokingly referred to ALT standing for "Anarchists, Lunatics and Terrorists" (a backronym).
In 1995, humanities.* was created for the discussion of the humanities (e.g. literature, philosophy), and the Big 7 became the Big 8.
The alt.* hierarchy has discussion of all kinds of topics, and many hierarchies for discussion specific to a particular geographical area or in a language other than English.
Before a new Big 8 newsgroup can be created, an RFD (Request For Discussion) must be posted into the newsgroup news.announce.newgroups, which is then discussed in news.groups.proposals. Once the proposal has been formalized with a name, description, charter, the Big-8 Management Board will vote on whether to create the group. If the proposal is approved by the Big-8 Management Board, the group is created. Groups are removed in a similar manner.
Creating a new group in the alt.* hierarchy is not subject to the same rules; anybody can create a newsgroup, and anybody can remove them, but most news administrators will ignore these requests unless a local user requests the group by name.
Further hierarchies 
There are a number of newsgroup hierarchies outside of the Big 8 (and alt.*) that can be found on many news servers. These include non-English language groups, groups managed by companies or organizations about their products, geographic/local hierarchies, and even non-internet network boards routed into NNTP. Examples include (alphabetically):
- aus.* — Australian news groups
- ba.* — Discussion in the San Francisco Bay area
- ca.* — Discussion in California
- can.* — Canadian news groups
- cn.* — Chinese news groups
- chi.* — Discussions about the Chicago area
- de.* — Discussions in German
- england.* — Discussions (mostly) local to England, see also uk.*
- fidonet.* — Discussions routed from FidoNet
- fr.* — Discussions in French
- fj.* — "From Japan," discussions in Japanese
- gnu.* — Discussions about GNU software
- hawaii.* — Discussions (mostly) local to Hawaii
- hk.* - Hong Kong newsgroups
- hp.* — Hewlett-Packard internal news groups
- it.* — Discussions in Italian
- microsoft.* — Discussions about Microsoft products
- pl.* — Polish news groups
- tw.* — Taiwan news groups
- uk.* — Discussions on matters in the UK
- yale.* — Discussions (mostly) local to Yale
Additionally, there is the free.* hierarchy, which can be considered "more alt than alt.*". There are many local sub-hierarchies within this hierarchy, usually for specific countries or cultures (such as free.it.* for Italy).
See also 
- The Big-8 Management Board
- Alphabetical list of Usenet hierarchies
- What newsgroups are and how they work - describes how newsgroups work; what happens to articles after they are posted
- Newsgroups Charter Directory - A charter list to educate and inform users about specific Usenet groups and their purpose