Neutral Density Filter
The Neutral Density filter is a solid, dark filter, much like a pair of sunglasses (but not polarized). It is used to reduce the amount of light that enters the camera.
A neutral density filter is useful any time you need to block light out of the composition, and is especially useful for shooting running water and waterfalls. Because of the reduced light, the shutter speeds are slower, giving the moving water that often sought after ethereal or dreamy look.
Split Neutral Density Filter
The Split Neutral Density Filter is simply a Neutral Density Filter that is opaque on one half and clear on the other half. When used in a composition, the clear area of the filter will not impede light from entering the camera, but the dark portion will “hold back” the light (the amount depends on the strength of the filter being used).
These filters are great for narrowing the range of tones in an image. For example, if you are shooting a landscape where the top of the composition is a snow-covered mountain, and the bottom of the image is a cabin in the shade, the range of tones is too much for the camera to accurately capture the entire scene. Either the snow will be blown out (when you expose for the shady area), or the shady area will be too dark (when you expose for the snowy area).
Using the graduated filter, with the dark half covering the snowy area, and the clear portion over the shady area, you can shrink down the range of tones. Split Neutral Density filters come in a variety of strengths, as well as either “hard” or “soft.” This refers to the gradation from clear to dark; the hard gradation goes immediately from clear to dark, while the soft fades from clear to dark.