Depth of field
This is the distance between the nearest and farthest objects in a scene that appear acceptably sharp in an image.
In shallow focus one plane of the image is in focus while the rest is out of focus. Shallow focus is typically used to emphasize one part of the image over another.
This is the front-to-back range of focus in an image — that is, how much of it appears sharp and clear. Consequently, in deep focus the foreground, middle-ground and background are all in focus.
The pull focus is useful for directing the viewer’s attention. For example, if there are two people in shot but only one is in focus, that person is the subject of attention. If the focus changes to the other person, they become the subject.
This is often used in drama dialogues — the focus shifts backwards and forwards between the people speaking. A slightly more subtle trick is to focus on a person speaking then pull focus to another person’s silent reaction.