Among the varieties of ND filters from ND2 to ND1000 even ND100000, we can generally categorize ND filters into 3 basic groups:
• Light density filters (ND2 - ND8)
• Middle density filters (ND16 - ND200)
• Deep density filters (ND500 - ND1000 / ND100000)
Light density filters will lower the shutter speed in the bright sunny day to allow you to reduce depth of field in very bright light or to use a wider aperture to stay below the diffraction limit. For example, these filters are necessary to shoot with your fast lenses like F1.4 or F1.2 with aperture wide open for softer and more beautiful bokeh.
If you are for more creative shots that are based on extremely slow shutter speeds - deep density filters are to be suggested. ND filters like ND500 or ND1000 will help to capture dramatic unreal fantasy world that cannot be seen by the human eye.