The Neutral Density (ND) filter is a great tool for photographers to have in their arsenal. It works like sunglasses for your camera lens, blocking part of the incoming light and allowing you to take better pictures in bright light. ND filters are also useful for creating longer exposures, blurring moving objects, and darkening part of the image. A clear ND filter can last one or two steps, while a very dense ND filter can last 15 or 20 steps.
Ten- and fifteen-step filters are great for removing moving objects from an image. Graduated filters are useful when you only want to darken part of the image, such as when the sky is brighter than the foreground. Square filters also have more surface area, which means more area of moisture accumulation when recording near the ocean, waterfalls, or in the rain. To use an ND filter, it is placed in front of the lens and blocks part of the incoming light. If there's a lot of light, you might be able to measure exposure through the filter, but if you're shooting at the end of the day, you might have to calculate the exposure manually.
A filter system can take a little longer to set up and, when the lighting is good, you want to take the picture, not fight with your equipment. Photographers who have extensive knowledge of Photoshop can compose an image using a quick shot that was taken without the ND filter and another that was taken with the ND filter. It's always a good idea for DSLRs to cover the camera's eyepiece, especially if the sun is behind the camera. Since portrait photographers tend to use a very small depth of field (open aperture), it may not be convenient to reduce exposure with the aperture and, if the ISO is already at maximum, the next best option is to use a one- or two-step neutral density filter. Once again, both have been exposed to exactly the same amount of light, but with the ND filter in place you can let light in slowly, which allows moving things to blur out, like clouds, and also allowing things to mix, like waves in water.