A neutral density filter has long been the staple of a landscape photographer and for good reason. While the effects of many other filters may be recreated in post-production, the ND filter is useful during capture as it extends the range of shutter speeds and apertures that can be to be used in a set condition, which in turn can be used for creative effect. So how does it do this?
As its name suggests, it filters different wavelengths of light equally, so that the resulting image shows no colour bias. This is important, as it is used for controlling the overall amount of light which reaches the sensor, as opposed to correcting any particular cast. Each filter is graded in terms of optical density, with each 0.1 factor reducing exposure by 1/3 stop. So, a 0.3 filter cuts down one stop of light, a 0.6 filter cuts two and so on. It's also possible to stack filters together, which allows you to maximise this effect, and thus the time which you keep the shutter open.
These apply their effect equally throughout the whole filter, although you can also buy graduated types which instead are clear on one side. These are useful for balancing exposure for landscapes, where a sky will be much brighter than the foreground detail which would ordinarily exceed the dynamic range of a sensor. As with standard ND filters these also come in different strengths, although they are also graded by how hard or soft they are - that is, how suddenly their gradation changes.