Soft (Mapped) shadows are generated using a shadow map generated at the same resolution as your rendering. Each light creates a shadow map and Point lights generate six shadow maps for each light. These shadows render fast but use a lot of memory. See the Memory Usage for each light in the Light Parameters. Adjust the softness of the shadow in the Soft Shadow Parameters. These shadow can’t create transparent shadows. Turn Cast Shadow attribute OFF if you want light to pass through objects. It is also recommended to turn Cast Shadows OFF for any large ground planes to increase the quality of soft shadows.
Hard (Raytraced) shadows are raytraced for each light. These shadows render a little slower than soft shadows but consume no memory. There are no softness controls. These shadows can create transparent shadows through transparent materials. You can turn Cast Shadows attribute OFF if you want light to pass through objects. Ground planes have no affect on hard raytrace shadow quality.