D for Depth of Field (DOF)
DOF is significant in deciding if a photo is amazing or little off! As the name suggests, DOF is the part of the photo, which is in sharp focus. Its not a single point, but rather a span, a vertical plane. The depth varies directly with aperture. Smaller F number, smaller DOF. If you want a portrait, a smaller DOF is suitable, and in a landscape, for having both foreground and background in focus, a large DOF i.e a large F number. Having a large DOF helps, if you miss pinpoint focus. More, here.