Neutral Density Filters are used to take photos in slower shutter speeds during daytime. They are usually black and gray in color. These filters can block greater amount of sunlight to pass through the lens. In the market, they are named as ND2, ND4, ND8, etc. to understand these numbers further, you may click this link: http://en.wikipedia.org/wiki/Neutral_density_filter
ND filter's prices' ranges from $20 to $100 depending on the brand and type of filter. The most commonly used is ND8 which transmits 12.5% of light into the lens. As for me i was eager to create my own ND filter first before buying an original one. The materials i used were: thick cardboard, number 11 welding glass and masking tape.
I took my first ND filter shot in a park near my place. To my surprise, a "greeny" filter effect across the photo was created. Final edit of the photo was made in adobe photoshop. afterall, i was happy with the result :-)
