Depth of field is the range of focus in a shot, so it is how far away something can be and still be in focus. 

Deep focus: A very lager depth of focus so the foreground, and background are both in focus at the same time. These are often used to give an overall picture of the scene.  

Shallow focus: A very narrow depth of focus so only one part of the shot is in focus and everything else is out of focus. These are often used to draw the viewer’s attention towards an object or person.