Electronic discussion groups consisting of collections of related postings (also called articles) on a particular topic that are posted to a news server which then distributes them to other participating servers. There are thousands of newsgroups covering a wide range of subjects. You must subscribe to a newsgroup in order to participate in it or to track the discussion on an on-going basis. Unlike with a magazine or newspaper, subscribing to a newsgroup does not cost anything.
Newsgroups are found primarily on Usenet. Usenet is the collection of computers that participate in a global conferencing system that make newsgroups perhaps the largest distributed bulletin board system in the world. Newsgroups are one of the oldest and most widely used services on the Internet. There are more than 13,000 of them, with new ones coming online all the time. Not all newsgroups are carried by Usenet, and Usenet is carried by networks that are not on the Internet.
They are divided into different categories, the seven official topics titled the "Big Seven" hierachy (comp, misc, news, sci, soc, talk). The other popular category is alternative (alt).