A newsgroup is an individual UseNet discussion forum relating to a specific subject or topic. Newsgroups are public discussion forums, in which anyone with a newsgroup reader can participate. Because newsgroups are typically frequented by those experience in the topic for which the newsgroup was set-up, they can be extremely useful sources of advice and help. Often posting a message to a relevant newsgroup can be one of the best means of getting help and advice.
Some newsgroups are moderated, others are not. A moderated newsgroup is one where either articles are posted to a moderator who reviews the article before it appears on the newsgroup (this is known as pre-moderation), or where a moderator reviews articles after they have been posted and has the authority to remove unsuitable postings (this is known as post-moderation).
cf What is UseNet.