Describe the difference between threaded and unthreaded discussions.
I assume by unthreaded you mean nested.
A threaded discussion is usually a simple chronological listing of comments. There is often no real delineation as to which replies go to which question or discussion topic. They show in order in which they were posted.
In a nested view the posted have top level posts and then some indicator (usually a + or -) next to it to expand or collapse the discussion to show the replies.
Hope this helps