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11.4 Terminology

news
This is what you are supposed to use this thing for—reading news. News is generally fetched from a nearby NNTP server, and is generally publicly available to everybody. If you post news, the entire world is likely to read just what you have written, and they'll all snigger mischievously. Behind your back.
mail
Everything that's delivered to you personally is mail. Some news/mail readers (like Gnus) blur the distinction between mail and news, but there is a difference. Mail is private. News is public. Mailing is not posting, and replying is not following up.
reply
Send a mail to the person who has written what you are reading.
follow up
Post an article to the current newsgroup responding to the article you are reading.
back end
Gnus considers mail and news to be mostly the same, really. The only difference is how to access the actual articles. News articles are commonly fetched via the protocol NNTP, whereas mail messages could be read from a file on the local disk. The internal architecture of Gnus thus comprises a “front end” and a number of “back ends”. Internally, when you enter a group (by hitting <RET>, say), you thereby invoke a function in the front end in Gnus. The front end then “talks” to a back end and says things like “Give me the list of articles in the foo group” or “Show me article number 4711”.

So a back end mainly defines either a protocol (the nntp back end accesses news via NNTP, the nnimap back end accesses mail via IMAP) or a file format and directory layout (the nnspool back end accesses news via the common “spool directory” format, the nnml back end access mail via a file format and directory layout that's quite similar).

Gnus does not handle the underlying media, so to speak—this is all done by the back ends. A back end is a collection of functions to access the articles.

However, sometimes the term “back end” is also used where “server” would have been more appropriate. And then there is the term “select method” which can mean either. The Gnus terminology can be quite confusing.

native
Gnus will always use one method (and back end) as the native, or default, way of getting news. Groups from the native select method have names like ‘gnu.emacs.gnus’.
foreign
You can also have any number of foreign groups active at the same time. These are groups that use non-native non-secondary back ends for getting news. Foreign groups have names like ‘nntp+news.gmane.org:gmane.emacs.gnus.devel’.
secondary
Secondary back ends are somewhere half-way between being native and being foreign, but they mostly act like they are native, but they, too have names like ‘nntp+news.gmane.org:gmane.emacs.gnus.devel’.
article
A message that has been posted as news.
mail message
A message that has been mailed.
message
A mail message or news article
head
The top part of a message, where administrative information (etc.) is put.
body
The rest of an article. Everything not in the head is in the body.
header
A line from the head of an article.
headers
A collection of such lines, or a collection of heads. Or even a collection of NOV lines.
NOV
NOV stands for News OverView, which is a type of news server header which provide datas containing the condensed header information of articles. They are produced by the server itself; in the nntp back end Gnus uses the ones that the NNTP server makes, but Gnus makes them by itself for some backends (in particular, nnml).

When Gnus enters a group, it asks the back end for the headers of all unread articles in the group. Most servers support the News OverView format, which is more compact and much faster to read and parse than the normal head format.

The NOV data consist of one or more text lines (see Motion by Text Lines) where each line has the header information of one article. The header information is a tab-separated series of the header's contents including an article number, a subject, an author, a date, a message-id, references, etc.

Those data enable Gnus to generate summary lines quickly. However, if the server does not support NOV or you disable it purposely or for some reason, Gnus will try to generate the header information by parsing each article's headers one by one. It will take time. Therefore, it is not usually a good idea to set nn*-nov-is-evil (see Slow/Expensive Connection) to a non-nil value unless you know that the server makes wrong NOV data.

level
Each group is subscribed at some level or other (1–9). The ones that have a lower level are “more” subscribed than the groups with a higher level. In fact, groups on levels 1–5 are considered subscribed; 6–7 are unsubscribed; 8 are zombies; and 9 are killed. Commands for listing groups and scanning for new articles will all use the numeric prefix as working level.
killed groups
No information on killed groups is stored or updated, which makes killed groups much easier to handle than subscribed groups.
zombie groups
Just like killed groups, only slightly less dead.
active file
The news server has to keep track of what articles it carries, and what groups exist. All this information in stored in the active file, which is rather large, as you might surmise.
bogus groups
A group that exists in the .newsrc file, but isn't known to the server (i.e., it isn't in the active file), is a bogus group. This means that the group probably doesn't exist (any more).
activating
The act of asking the server for info on a group and computing the number of unread articles is called activating the group. Un-activated groups are listed with ‘*’ in the group buffer.
spool
News servers store their articles locally in one fashion or other. One old-fashioned storage method is to have just one file per article. That's called a “traditional spool”.
server
A machine one can connect to and get news (or mail) from.
select method
A structure that specifies the back end, the server and the virtual server settings.
virtual server
A named select method. Since a select method defines all there is to know about connecting to a (physical) server, taking the thing as a whole is a virtual server.
washing
Taking a buffer and running it through a filter of some sort. The result will (more often than not) be cleaner and more pleasing than the original.
ephemeral groups
Most groups store data on what articles you have read. Ephemeral groups are groups that will have no data stored—when you exit the group, it'll disappear into the aether.
solid groups
This is the opposite of ephemeral groups. All groups listed in the group buffer are solid groups.
sparse articles
These are article placeholders shown in the summary buffer when gnus-build-sparse-threads has been switched on.
threading
To put responses to articles directly after the articles they respond to—in a hierarchical fashion.
root
The first article in a thread is the root. It is the ancestor of all articles in the thread.
parent
An article that has responses.
child
An article that responds to a different article—its parent.
digest
A collection of messages in one file. The most common digest format is specified by RFC 1153.
splitting
The action of sorting your emails according to certain rules. Sometimes incorrectly called mail filtering.