| Hi Arquebus, 
Planar creates a flat surface from a closed curve that is "planar" 
 meaning it's flat to a particular plane as opposed to being curved in 
all three planes, X Y and Z.  Also, if you have, let's say a box with an open side, 
 Planar will cap it with a flat surface because the edges of the 
 open side of the box are all on the same plane.
 
Blend puts a transitional surface between two surfaces, or a transitional curve 
 between two curves.
 
I just discovered something cool..Blend has history turned on...so if you move the construction curve of one of the 
 originating surfaces of a blended surface, the blend surface stretches to accommodate 
the movement of the originating surface.
 
  |