Right now the only differences I can see is:
You can get a NG in a newsreader like my Forte Agent and you use a different address like
NEWS.MICROSOFT.COM
You can always access a newsgroup with a newsreader.
although you can also access it from the web address.
Maybe. It depends entirely on the newsgroup. Some (like this one) have
web access. Others don't.
A forum always has web access, but not all have newsreader access.
Is there any other differences?
Again it depends on the forum. Some let you upload and download files,
others don't.
Newsgroups permit offline access, forums don't. Even if you're reading
newsgroups online, in a newsgroup you can read one message while
others are downloading; in a forum you generally download each message
just before you read it, so you have to wait for it and it's much
slower.
Newsreaders have filtering capability that lets you avoid wasting time
reading messages from trolls and spammers. Conversely, it lets you
mark the messages of posters you especially want to read, so they are
highlighted.
Newsreaders let you sort messages in the order *you* want to read them
in.
Newsletters let you mark a thread you're interested in, so you can
quickly see any new messages in that thread.
Newsreaders generally have spell checkers; forums generally don't.
And so on. Try both and you should quickly see the many differences.