Depth of Field
Depth of field is basically what will be in or out of focus in your picture. There are 3 basic factors that determine how much or how little depth of field. Firstly aperture, the smaller the aperture, the more extensive the depth-of-field. If you want to keep as much as possible sharp, you should set as small an aperture as possible. If, however, you want to concentrate attention on just one part of the scene, and throw the rest out-of-focus, you should select a large aperture. Exactly how large this can be depends on the maximum aperture of the lens you're using. On a 50mm standard lens it will be f/1.7, f/1.8 or f/2, but on a standard zoom it will typically be f/3.5 or f/4.5. Secondly, the shorter the focal length (the wider the lens) the greater the depth of field is at any given aperture. Thirdly, the further away the main point of focus is from the camera, the greater the d-o-f will be for any given lens and aperture setting. One thing to remember is a low f stop (number) eg F2.8 gives a large aperture, while an high number eg F8 gives a smaller aperture.
The most common use for shallow depth of field is portraiture where you want to emphasise the subject.
Deep dof is more flexible but a good example of when to use it would be landscapes photos.
Full Depth Of Field
Shallow DOF 12/10/12