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.