First, forget the word "sites"; this has nothing to do with the Web as you know it.
Usenet, which dates back to 1980 or so, is a distributed message-board system with no central server: any individual location can carry any actual newsgroup. It uses NNTP (Network News Transfer Protocol). You can read some of the stuff through Google Groups, but you'll need an actual account with a newsgroup provider -- which will cost you, since ISPs quit giving it away years ago -- to do any serious work.
The provider I use is Agent, which also offers a newsreader by the same name. (It is not necessary to use Agent's newsreader to use Agent's service.) If you use Windows Live Mail or Mozilla Thunderbird mail clients, they can also be used to read newsgroups.
Generally, there are two types of messages in Usenet: text, which is readable, and binaries, which are encoded to fit in a text distribution but can be decoded to another file type (typically, pictures). Your newsreader should do the decoding on the fly.