Usenet is an online bulletin board system that began at Duke University in 1979. Usenet users can post messages to newsgroups that can be read (and responded to) by anyone who has access to the system through a newsreader. Over the years, the number of newsgroups has grown into the thousands; they're hosted all over the world and cover every conceivable topic.
Google Groups contains the world's most comprehensive archive of Usenet postings, dating back to 1981. Using Groups, you won't need a newsreader and you can search this archive the same way you'd search on the web. You can also use Groups to post your own comments to an existing Usenet newsgroup.