Newsgroups are subsets or categories of Usenet that are electronic discussion forums for sharing information and opinions with people on thousands of different topics. Newsgroups allow you to read articles as well as post or respond to articles.
You can think of a newsgroups as electronic bulletin boards organized by topic. If you want to say something, you "post" it, if you want to "read" something, you read it with a newsreader. However, you have to make sure that you're reading and posting information in the right areas. You shouldn't post something about soccer on the basketball bulletin board, for instance. Some of the bulletin boards have moderators, that keep an eye on the board to make sure only the relevant and proper information is posted, while others are unmonitored and therefore have an "anything goes" approach.
Newsgroups are grouped by topic and title using names such as rec.sport.basketball.college. Here, rec specifies recreational topics, sport specifies a subgroup of recreation, and so on. More specifically, categories include:
Many subcategories are listed under each of the main categories; under comp, for example, you'll find the subcategory comp.sys.mac.
The first time you use Usenet, it's highly recommended that you read the newsgroups news.newusers.questions and news.announce.newusers to learn about technique and "netiquette" before posting.