Stack n Whack are made with 4, 6 or 8 or more identical stacked repeats and can be squares (4), hexagons or half square triangles (6), octagons (8) or more (like dresden plates) and are usually made into blocks with sashings or other fabric between the actual stack n whack blocks. Stack n Whacks can be made with one or more different fabrics in the same quilt.
One Block Wonders (OBW) on the other hand are hexagons using identical stacked repeats of the one fabric all joined together without sashing or other fabric between the blocks.
I hope that makes sense :?