The area of sharpness in a picture, extending in front of and behind the plane of the subject, that is most precisely focused by the lens.

You can control depth of field by varying three factors:
1) the size of the aperture
2) the distance of the camera from the subject
3) the focal length of the lens.

If you decrease the size of the Aperture, the depth of field increases; if you focus on a distant subject, depth of field will be greater than if you focus on a near subject; and if you fit a wide-angle lens to your camera, it will give you greater depth of field than a  normal lens viewing the same scene. Many SLRs have a depth of field preview control - a button that closes the lens diaphragm to the Aperture selected for an exposure so that the depth of field in the image can be checked on the viewing screen first.