Depth of field (DOF) is something that a lot of people do not have a good enough grasp of to be able to take advantage of. If used properly one is able to isoloate a subject so it is the primary thing a person will see. Depending what you are photographing will dictate how much DOF you need to use.
Let's cover the items that control the DOF in an image.
Aperture size
The larger (smaller number) that your aperture is the less DOF you will have to play with. If you take the same image at f/2.8 vs. f/22 you will see more of the background will be visible at f/22 compared to f/2.8. Knowing this will let you control how much is in focus.
Lens Focal Length
The longer your lens is the less DOF you have. This is the main reason most landscape photographers use wide angle lenses so that mountain in the distance is nice and sharp along with the river in front of them.
Distance from subject
This one has to be the one that most people do not realize affects your DOF. As you get closer to the subject less of the background will remain in focus. Conversely as you step away more will be sharper.
Putting all this together is the image below. It was taken with a Nikon 70-200mm VR lens set at 200mm at f/2.8 at about 6 feet away (2 meters for the rest of the world) Notice that the background is completely out of focus. The barbed wire really stands out in this image.
Here is another post from a soccer/football blog that really illustrates great DOF.
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