Depth Of Field

Depth Of Field- depth of field (DOF) is the distance between the nearest and farthest objects in a scene that appear acceptably sharp in an image. 
Although a lens can precisely focus at only one distance at a time, the decrease in sharpness is gradual on each side of the focused distance, so that within the DOF, the unsharpness is imperceptible under normal viewing conditions.  In some cases, it may be desirable to have the entire image sharp, and a large DOF is appropriate. In other cases, a small DOF may be more effective, emphasizing the subject while de-emphasizing the foreground and background.

This image is a great example of DOF.