Before the World Wide Web, USENET was the largest single source of Internet activity. Even now it makes up a good portion of the overall traffic.
It's a place where you can find discussions on pretty much every topic imaginable. Unfortunately, it's also become a huge source of unwanted advertising.
Below are some links to help get you started, but the best thing to do is just follow some newsgroups that you're interested in for a week or two and then jump in.
Newsreader Setups
How to set up various supported newsreaders for
StarNet
USENET Netiquette
Dos and Don'ts of USENET and the Internet in
general
Binary Files in Newsgroups
An introduction to files transmitted through
newsgroups
Decoding Binaries in Newsreaders
General guide to extracting binary files using
various supported newsreaders.