What is Depth of Field ?

It's the portion of a scene that appears sharp and in focus.

photography.about.com

Middle swan is sharp and in focus.

Related: Aperture.

Depth of Field	digital-photography-school.com
Small Aperture, large DOF, everything sharp.	Large Aperture, small DOF, background blur.

References:

• http://www.cambridgeincolour.com/tutorials/depth-of-field.htm
• http://en.wikipedia.org/wiki/Depth_of_field
• http://photography.about.com/od/takingpictures/ss/DOF.htm