D is for Depth of Field
Depth of Field refers to the depth of the area of a photograph which is in sharp focus. A photographer can control the depth of field using the aperture setting or f-stop when taking the photograph.
Portrait photographers use a shallow depth of field (large aperture) to throw the background (or sometimes foreground) out of focus. This helps the viewer understand the focal point of the photograph.
Landscape photographs more usually use a small aperture (f11 or f16) to create a deep field of focus and make sure all elements are in focus.