Conceptual différence between "activities" and "actions"

I don't really get the conceptual difference between "activities" and "actions".
I tried to catch information in the online user guide, with no success.
I tried the same by searching in the forum, with the result.

Is the action the atomic unit for activities?
Are the two objects completely separate?
In which situation do u use one rather than the other? - especially in the planning and Gantt views

Anyone who use projeQtor in real life is welcome to provide me with some feedback.
Thank you