A Neutral Density Filter is one designed to block out a substantial amount of light. This enables the photographer to slow down his shutter speed and/or shoot at a faster F-stop.
Most photographers will use what is called an N2, N4, or N8 filter to smooth out waterfalls, rivers, oceans. This photographer has used an N64 filter which is far stronger, letting him have an exposure of 60 seconds at f/16 for this image. Without the filter, the photographer would have had to use f/2 to get away with a 60-second image, or less than a second at f/16.
An N64 filter allows for 1.5% of available light to enter the lens. Here is comparison table for the other filters, courtesy of wikipedia:
Filter Name F-Stop Reduction % of Light Transmitted
ND2 has an F-Stop Reduction of 1 which means 50% of the light gets through
ND4 has an F-Stop Reduction of 2 which means 25% of the light gets through
ND8 has an F-Stop Reduction of 3 which means 12.5% of the light gets through
ND64 has an F-Stop Reduction of 6 which means 1.56% of the light gets through
ND1000 has an F-Stop Reduction of 10 which means .1% of the light gets through
Subject: Neutral Density Filters 
Sun Feb 14, 2010 11:50 am