ND filters can be used to create motion blur effetcs on water for one.


This is done by increasing the exposure time (shutter speed) allowing more light to enter and create a blur, but then the ND filter brings down the exposure so that the image isn't over-exposed as well.


ND filters come in different strengths, for example: ND2, ND4, ND8


Each srength becomes stronger (darker)


ND filters can also be used for portrait shooting for decreasing exposure as well.