Depth-of-field (DOF) refers to how much of a photo is in focus when the camera is focused on the main subject. Depth-of-field is controlled by a camera's aperture, in conjunction with the focal length of the lens.
 

Maximum DOF

Maximum depth-of-field means that all or most of the picture is in focus from front to back. It is often used for capturing subjects in the distance, such as in landscape scenes. The further away your camera is from the subject, the greater the depth of field. Higher f-stop numbers obtained by using smaller apertures produce deep depth-of-field. Short focal lengths (when lens is set at wide angle) help increase depth-of-field.
 

Minimum (shallow) DOF

Minimum depth-of-field means that a subject is in focus but objects in front and behind it appear out of focus. Shallow DOF is often desired when you take portrait and close-up and macro photographs. Lower f-stops (larger apertures) decrease DOF. Long focal lengths (zooming in) produce less DOF. With a digital camera, you can also make the background appear out of focus by placing the subject close to the camera and having the background far away.