When talking about interleaved and PlanarImageFormat, the main difference is how they are stored in memory.

Interleaved images have all color components needed to represent a pixel placed at the same place in memory.

Some examples of interleaved formats are RGB24, RGB32, YUY2.

http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dnwmt/html/YUVFormats.asp