I don’t do a lot with an ND filter, so I only have one. It is adjustable. The advantages are it takes less space in the bag and you can adjust it to a low density value so you can see to focus, then adjust it to a greater density for the photo. I suspect dedicated density filters actually do a better job, but for playing around the variable one is enough.
A stop is half or twice the light. Half or double shutter speed, half or double ISO, or adjust aperture.
Here is a link to a discussion about ND filters: http://www.picturecorrect.com/tips/neutral-density-filter-tips/
Have to run. Hope this is enough to get you started.