Deep and Shallow Focus are part of the composition of a shot. They both are used to create different effects in a film.
Deep focus is where the whole image is in focus -
Here is a example taken from the film 500 Days of
Summer, starring Joseph Gordon-Levitt and Zooey Deschanel. Here all of the shot is in focus.
However Shallow focus is the opposite. One object or part of the shot will be in focus whilst the other is not. Typically its used to emphasise one part of the image over the other. Below again is an example from 500
Days of Summer. It shows Summer (Deschanel) in focus, whilst Tom (Gordon-Levitt) and the background are out of focus to emphasised Summer and her actions.
I wanted to look at deep and shallow focus because I was thinking about how we might be able to use them in our opening sequence, perhaps to emphasize important parts of the scene.