This is what you are supposed to use this thing for - reading news. News is generally fetched from a nearby NNTP server, and is generally publicly available to everybody. If you post news, the entire world is likely to read just what you have written, and they'll all snigger mischievously. Behind your back. Mail is private.
News is public.
Mailing is not posting, and replying is not following up. Gnus gets fed articles from a number of backends, both news and mail backends. Gnus does not handle the underlying media, so to speak - this is all done by the backends. You can also have any number of foreign groups at the same time. These are groups that use different backends for getting news.