It\'s just kinda like a forum, but only with text (no graphics). So basically it\'s one BIG forum with 40,000+ different groups (topics), where you discuss, buy and sell, or whatever the group is for. It can be good fun and quite useful.
BTW, if you\'re searching for something in particular Google.com has an option to search newsgroups, and then show the messages like ordinary links (if it sounds confusing, just try it out - search for Gran Turismo 3, or whatever, to try it).