One of Usenet's huge collection of topic groups or fora. Usenet groups can be "unmoderated" (anyone can post) or "moderated" (submissions are automatically directed to a moderator, who edits or filters and then posts the results). Some newsgroups have parallel mailing lists for Internet people with no netnews access, with postings to the group automatically propagated to the list and vice versa. Some moderated groups (especially those which are actually gatewayed Internet mailing lists) are distributed as "digests", with groups of postings periodically collected into a single large posting with an index.
Among the best-known are comp.lang.c (the C-language forum), comp.arch (on computer architectures), comp.Unix.wizards (for Unix wizards), rec.arts.sf-lovers (for science-fiction fans), and talk.politics.misc (miscellaneous political discussions and flamage).
See also netiquette.