I posted this question about the differences of directories vs folders on SO some time ago. Despite my effort, it was migrated to SU.
Since I strongly believe that this question should stay on SO, I flagged it, only to have it closed as a dupe of a SU question that even got a wrong answer accepted.
Flagging again is not possible, so I'm putting this issue up here. I'm a bit sad about how this was dealt with so far. The question content is definitely relevant to programmers, and I already stated that I'm explicitly not talking about the usual user POV.
Edit: Why the question belongs on SO
Any programmer that deals with the file system or the shell on e.g. Windows is going to work with files. It is, as seen in my answer quoting Raymond, a huge difference if your program handles folders or directory - for instance, a directory-based program won't be able to work with files inside a zip file. You also won't easily be able to tamper with any other kind of virtual files, like the recycle bin or the control panel items.
Certainly, users (since we're on SU) may be interested in the difference as well, but as stated I'm explicitly talking about the programmer POV, and a programmer will look for this question on SO and not on SU. You're bound to get dupes if this question can't be found on SO.
Therefore, I strongly believe that knowledge of folder/directory differences is important to programmers, and the question ought to be moved back.