A planar image is one that renders shadows as a solid dark color, and lighter parts as a solid light color or white, with no shading (1-10). A silhouette is similar but uses only the contour of the whole subject and disregards any light falling on it (11-20).
(Excerpt from Logo Theory: How Branding Design Really Works,
Section 29: Visual Technique #2: Planar or Silhouette)

Buy Paperback from Amazon.com click here

from BarnesAndNoble.com click here

from Amazon.ca
click here

from Chapters.ca
click here

Buy for iBooks
click here

Buy for Kindle
click here