This 20 minute video provides a detailed explanation of depth-of-field and the factors that affect it. This tutorial was designed specifically for filmmakers who use or are planning to use 35mm depth-of-field adapters. The information can also be applied to still photography and other suited mediums.
This may be the best DOF description I've ever seen, and is essential viewing for all you photogs out there, even if you already know much of the material. (Via Richard Wanderman.)