Shot VS Setup

 What is the main difference between a shot vs. a setup?   What is the most important thing to keep in mind when you're going to shoot a scene?

The setup is the lights, position of actors, and the way all the props are set. Frequently, you want to shoot out of order to save time, and avoid changing your setup. That's the most important thing. A shot is the actual clip you want to shoot. Your setup is for your shot.