[brianogilvie]

To what ptone wrote, I would add:

A project is something that takes more than one physical step (=action) but that has a definite "done" state. Example: review a book. (When I've sent the review to the magazine or journal that's publishing it, it's done.)

A folder is an area of responsibility that will never be done unless your life changes. For me, "Book Reviews" is an area of responsibility. I'll finish individual book reviews, but I'll never finish reviewing books until I retire (or I decide that the return doesn't compensate the investment, but I hope I never get there!).

An action group is like a lemma in a mathematical proof: it's a way of organizing related actions whose completion will move you toward finishing a commitment (project) but that won't get you there on their own. In my book review case, I might have an action group for "draft and revise review" with the various stages of writing and revising in it.

I hope that helps!