Neutral gradient filter

[PKMax]

an ND filter (neutral Density) are designed to prevent some light getting into the camera, which will allow you to do things like extend the shutter time when it's too bright to do time exposures. Like if you wanted to do a time exposure during the day in a street to blur all the people walking past, but there is too much light to be able to get the shutter speed slow enough even with the aperture on the smallest setting. so adding one or more ND filters will cut down the light coming in and require that you increase your shutter speed to compensate.

Then you have the ND Gradient, which is similar to above but it's a gradient from dark through to clear, half and half of the filter. the reason for these are so that you can take a landscape for example, where the sky is too bright, so exposing for the ground gives you an overexposed sky, and exposing for the sky gives you a underexposed ground. so using a ND gradient you can cut down the light coming in through the half that you want to underexpose a little (like the sky) and have a more even exposure.

also, the markings on an ND filter would be something Like ND 1, ND 4, ND 8 etc.. where the number indicates their strength (larger number, the darker it is) I think the number relates to the Density of the filter.. but that number translates to an fStop reduction somehow. ie ND1 is a about 3 stop reduction, ND4 is a 13 stop reduction.

Polarizing Filters to have the effect of cutting down the amount of light that comes into the lens, but this is just due to the way they work.. they "convert" unpolarized light into polarized light, Light waves generally travel in all angles, especially when being bounced off all those objects you see. so a Polarizing filter allows you to remove the light that is travelling at a certain "plane" so when bounced of water the beam will be travelling towards your camera in a horizontal fashion, so turning the polarizing filter can prevent this light getting into the lens. it's easy to see the effect of a polarizing filter by turning it around in front of an LCD monitor, because they only put out polarized light. so turning the polarizer to a point will cause the screen to look black through the filter because it will have filtered out all light coming from the monitor.

Polarizers are used to reduce the glare from things like water, glass. and also they make skies blue-er and colours generally more saturated.