Wikipedia defines Depth of Field (DOF) as follows:

“Depth of field (DOF) is the distance between the nearest and farthest objects in a scene that appear acceptably sharp in an image.”

There are two types of DOF

Shallow – Foreground is in focus and background is out of focus
Great – Objects in the foreground and background are in focus

Examples of Shallow DOF --

Example of Great DOF --

Below is a set of shots in which I changed the aperture each time affecting the depth-of-field. Notice how more and more of the pumpkins come into focus the lower (smaller) the aperture gets.

f/2.8 f/5.6 f/8

f/11 f/16 f/22

Shallow depth-of-field is very useful when you want your main subject to be in focus and the background not in focus. This leads the viewer’s eye to main part of the photo.

I hope, after this, you have a little better understanding of depth-of-field.

"The more you understand what inspires you, the more readily you can put yourself in it’s path." - David duChemin