Folders are 'static': new entries are filed after older entries,
and there is no threading nor expiration. You'll have to remove
entries by deleting them manually. They behave like simple
filing cabinets. Pseudo newsgroups are threaded and they can
be expired. Incoming mail can be filtered to both. If you
plan on just saving that mail, you can use a folder. If you
want to treat that mail (esp. from mailing lists) like you
treat news articles, use a pseudo newsgroup.