Grey Graduated Filter:
It is a half colored filter which, when fitted to the lens, darkens one part of the scene. Especially suited to skies, the graduated filter can be used to darken them and retain detail in areas which would otherwise be bleached out throw overexposure. The grey graduated variety is the most versatile of graduated filters, and comes in varying strengths, from 1 (lightest) to (3) darkest).
When using a graduated filter always take the metre reading before the filter is attached, this will maximize its effectiveness.

http://www.pro-design.com.au/cam-pg2.htm

ND filter:
Neutral Density filters are often ignored by photographers, but they have several uses and offer the possibility to achieve otherwise unachievable results.
ND filters appear grey and reduce the amount of light reaching the film , they have no effect on color balance. They have 4 main uses:
 To enable slow shutter speeds to be used, especially with fast films, to record movement in subjects such as waterfalls, clouds, cars, seas, etc.
 To decrease depth of field by allowing wider apertures to be used, which helps separate subjects from their background.
 To decrease the effective ISO of high speed film and allow it to used outdoors in blight situations.


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((Half ND))