1. Here's the difference

You can apply an Effect to a single Clip or to the Entire Movie.

Effects that are assigned while Entire Movie is selected will apply to all Clips.

The exception to this rule is if you have also applied the same effect to one of your Clips.

In this case, the effect in the Clip will override the effect that is assigned to the Entire Movie.

TIP: having effects in the Entire Movie gives you much greater effectiveness in creating your Project.  You only need to edit the effect once for it to apply across all Clips.