Focus

Depth of Field
Check out the Wikipedia article on the subject:

Depth of Field

Compression of the Angle of View ("fake" Depth of Field)
Often mistaken for Depth of Field, a perceived shallow depth of field can be achieved by zooming in with a zoom lens or using a lens with a greater focal length (for example using a 100mm instead of a 50mm). However, when compressing the angle of view by zooming or using a higher focal length lens, distance between the subject which is in focus and items in the background and foreground appears closer. Thus while giving the appearance of a shallow depth of field, compressing the angle of view is different in this respect. The following article describes this phenomenon in great detail: Depth of Field Myth

Utilizing Depth of Field
Here's a great article on the subject:

DSLR Video Shooter's "Using Depth of Field for Storytelling"