[ptone]

This is going to be very subjective.

Basically: Folders are for related (or similar) but destinct projects (example "Work", "Home", "Repairs", "Products")

Projects are the "Core" collection object, it is the goal for which you wish to track actions.

And action group is useful if you have several actions that either have more detail then you always want to see, or seem to belong bundled together (maybe you would do them in rapid succession).

Anyway, there is no right "Answer" to your questions. You could easily use OF without using Folders or action groups, both are enhancements to the primary tool which is projects.

-P