A newsgroup is an Internet-based discussion about a particular topic. These topics range from sports, cars, investing, teen problems, and some stuff you probably don't want to know about. Users post messages to a news server which then sends them to a bunch of other participating servers. Then other users can access the newsgroup and read the postings. The groups can be either "moderated," where a person or group decides which postings will become part of the discussion, or "unmoderated," where everything posted is included in the discussion.
To participate in a newsgroup, you must subscribe to it. It typically doesn't cost anything, but some groups can be hard to get into unless you know people in the group. Nearly all newsgroups are found on Usenet, which is a collection of servers around the world. Because of the global spectrum of newsgroups, they make up largest bulletin board system (BBS) in the world. Last time I checked, there were more than 13,000 newsgroups in existence, with new ones being added all the time. You can choose from a number of different "Newsreader" programs that allow you to access and participate in newsgroups. Newsgroup access has also been integrated into Netscape and Internet Explorer, so you can just use your Web browser if you want.