Depth of field refers to the amount of space around the subject that is in focus. It can be used in many different ways to make a photograph interesting.
A shallow depth of field, where there is little space around the subject that is in focus may be used to isolate the subjects from a busy environment and bring extra attention to the subject. A shallow depth of field may also be effective when used to isolate a single object from a line of identical objects, like a single car from a long line of cars in a parking lot.
A shallow depth of field is archived when the aperture is fully opened. This is achieved by adjusting your camera to the highest f-stop (ex. f 2.4) and adjusting your shutter speed to compensate.

The opposite of a shallow depth of field would be a deep depth of field. It is used to give the viewer a broad understanding of the subject and it's setting. When photographing with a deep depth of field most of the frame will be in focus and the subject may not stand out or be noticed. It may be difficult to create a photograph with a deep depth of field that does not have distracting elements, but when effectively used, it will create visually pleasing photos.
A deep depth of field may be effective in a scenario where a photo of a person is being taken and at there are many other people looking at that one person. A deep depth of field will direct the viewer's attention to the subject because of the other people who are looking at the subject.
A deep depth of field is achieved by adjusting the aperture to a low value (f 14) and adjusting the shutter speed to compensate.
