The primary control of depth of field is the aperture, or f-stop, setting on your camera. Apertures range from f/1.8-f/64 and each lens you place on your camera will have a different aperture range which is indicated on the lens itself.
The depth of field (DOF) is the front-to-back zone of a photograph in which the image is razor sharp. As soon as an object (person, thing) falls out of this range, it begins to lose focus at an accelerating degree the farther out of the zone it falls; e.g., closer to the lens or deeper into the background.
Depth of Field is the area between the maximum and minimum distances from a camera to objects in a scene that can be captured in acceptably sharp focus. -or- Depth of Field is the total distance, on either side of the point of focus, appears to be sharp in the final print.