Neutral density filters are like sunglasses for your DSLR. They are primarily used for the purpose of balancing the exposure across the frame. They are imperative when you are about to shoot long exposures on bright, sunny days. They are also imperative for capturing creative long exposures in other lighting conditions.
You may have seen the ubiquitous silky smooth waterfall picture or may be a sunset at the beach with the tide coming in looking like a mist. Chances are that those pictures were taken using neutral density filters.
Neutral density filters are made of glass and are coated to reduce the amount of light that comes through. They are called neutral density because they stop all wave lengths of light equally across the frame without any prejudice.
Neutral density filters are available in a wide variety of types and formats. Some such as the Grad NDs (Graduated Neutral Density filters) are coated so that they stop light in varying degrees across the surface of the filter. Then there are the hard stop versions of neutral density filters which has a clear part that allows light to pass through and a coated part that stops light.
Neutral density filters can be further segregated on the basis of how you attach them to the lens. First and probably the most widely used are the screw-in types. These are simplest to use as all you need is a filter that has a thread specification that matches with that of the lens and you can simply screw it in place to use. Second is the slot-type, the most popular example of which is the Cokin P series filters and their adapter rings. Here you will need to first attach the filter adapter / holder. Afterwards you can slide-in a square filter. A critical advantage of these filters is that it is easy to slide in a new filter and or stack additional filters as required without hassles.
Variable neutral density filters are a new entrant into the world of filters and they are extremely convenient too for users who have to deal with the constantly changing light outdoors. These filters can be screwed on to the front of the lens as normal circular neutral density filters. However they have a front rotating section that can be adjusted by slowly turning it to change the blocking power of the filter, thereby stopping / allowing more light. It evidently eliminates the need to carry several different neutral density filters with different light stopping capacity.
Neutral density filters don’t add anything extra to your frame. Whatever you wish to capture the ND filters only make sure that the highlights are not blown out and details in the shadows are retained so that what you get is a perfect Kodak moment. One final tip when using ND filters, especially when taking long exposures, is to ensure that you have metered the shot properly and then changed the exposure value accordingly. This will ensure that the image is neither washed out nor under-exposed.