Planar images are stored with each color component separate.
Some examples of planar formats are I420, YV12.
For more information these URL's:
http://www.fourcc.org/fccyuv.htm#Planar%20YUV%20Formats
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dnwmt/html/YUVFormats.asp