The same term being used in two different areas, let the confusion ensue!
There are five core sharing areas in a Ning social network: Blogs, Forums, Videos, Photos, Groups.
The term "discussion" is made confusing because it is used in both Groups and Forums. They are however completely separate from each other. I don't have any real 'super-tip' to telling the difference, but just keep in mind they are separate. Group discussions will not appear in the Forum discussions; and Forum discussions will not appear in the Group discussions.
Here are the most visible difference to help distinguish:
Another difference is that discussions in the forum area allow you have a category.