The amount of distance between the nearest and farthest objects that appear in acceptably sharp focus in a photograph. A preferred selection Depth of field ("DOF") in a focused subject in an image can be quite subjective. Remember this, adequate selection of DOF for one situation, application may be unacceptable for another photographer.
In simpler term, we define depth of field as the zone of sharpest focus in front of, behind, and around the subject on which, when lens is focused on a specific subject; with TTL (through the lens) SLR camera, DOF can be previewed in the viewfinder of a camera - the preview is very handy for critical type of work. For an example, when taking a product shot, when you require absolute certain if DOF is adequate to cover the object you intend to photograph Generally, the closer the subject to the camera, the more evenly with the distribution of depth of field in front and behind the subject. As distance of focus extends, DOF usually will be more behind than in front of the focused area.
A typical example of a photo with shallow depth of field control. (only the main subject of interest is enhanced by throwing other elements out of focus.)
A typical example of a photo with extended depth of field control (From near to far in sharp focus)
By Beatrice Chong Jia Chien
Source: http://www.mir.com.my/rb/photography/fototech/htmls/depth.html
