Depth of field is the range of acceptable focus on a shot or photo- it refers to the area in front of the camera lens that is sharp and in focus. Selective focus draws the audiences‘ attention on a specific person, place or object.
An object of person closer to the camera might be solely in focus if the background contain unimportant information/imagery.
Bokeh can also be used to portray the emotions of a person, or atmosphere of a place. It should not be used unnecessarily and only when it truly adds to the scene or image.
Shallow Depth of Field= where part of the frame is soft or out of focus
Deep Depth of Field= where the entire frame, from the foreground to the background, is in focus
Aperture, focal length and focus distance affect depth of field:
This affects depth of field the most:
-the wider the aperture, the more shallow the depth of field
-the narrower the aperture, the deeper the depth of field
The longer the lens, the shallower the depth of field is.
The closer the focus is to the camera, the bigger the depth of field is
(http://www.elementsofcinema.com/cinematography/depth-of-field.html)