The Usenet is a collection of newsgroups, similar to bulletin boards, where people can post and read messages posted by other people. People hold conversations by posting a message, and responding to other people's messages.
There are A LOT of newsgroups (probably well over 30,000 at any one time). More are added and deleted every day. Each group has a name that reflects it's topic. At the beginning of each name is a general category, and the name gets longer with each specific category title. Like here are a few:
If you have any Newsgroup reader installed on your computer, then clicking on the names off the groups above will take you to view their messages, and let you post messages of your own. Both Netscape and Internet Explorer come with newsgroup reader software, but your Internet Service Provider (ISP) might not support the use of newsgroups.