Neutral Density Filters are using to match exposure of the shot on pre-exposed scenes or adding motion effect to scenes. Neutral density means that wavelengths of light are reducing equally. Final effect is more depth of field, light balance or motion blur effect without colourcast.
The reverse neutral density filters (or RGND) produce a neutral grey effect and are used to reduce the light in the photo, this allow the photographer a better control of the exposition and the deph of field without any color cast. The reverse filters allow you to shot the sun going down or the sunrise.