Newsreader is, as said, a program to read Usenet discussions. They come in all flavors and have different sorts of stripes on the doors, but usually, their basic purpose is to let people post, reply, followup, find and save articles. They often talk to the news server with NNTP, and may provide offline reading capabilities (but sometimes this is provided with external programs like Leafnode or slrnpull).
There are also web-based interfaces to the Usenet (such as Google Groups).
Some examples of newsreaders:
Following are advertised as such, but IMHO they should not be used for reading news...
- Pine - actually a MUA, not really good for reading news - though I've heard it now supports things like threaded newsgroup view too, which was previously not there. Use only if you've got nothing else. Same goes for Lynx, which nominally supports news reading.
- Microsoft Outlook Express - also a MUA, known to post stuff as base64 and do all sorts of other, um, interesting things.
- Netscape Communicator - ditto. (Well, by-default HTML isn't as annoying to read as base64... =)
... but if you don't believe in my IMHOs, you should take a look at GNKSA evaluations (at http://www.gnksa.org/) - these usually provide objective analysises about their quality and suitability for use.