Depth of Field (DoF) is the range in front or behind the point of focus where objects remain sharp and in focus. The wider the aperture
(lower f-number), the shallower the DoF. As such, depth of field plays a crucial part in the look and feel of an image.
Other factors than can affect the depth of field are lens focal length, distance between camera and subject.
See Large DOF
and Shallow DoF
for more details.