1. poliarising filter - it works to cut off the orthogonal plane of electromagnetic waves (light). In practical term - when used at 90 degrees from the light source it works wonder with blueness of sky and details of clouds.
2. MD - not sure what you mean. UV> Many photographers use UV filter (transparent) to cut off UV rays which may cause fogginess in images but silicon sensors (CCDs and CMOS) are not as sensitive to UV rays as silver nitrate based photographic film hence the effect may not be that obvious. The filter, however, is good just as a protector for lenses.
3. ND - neutral density - essentially acts to reduce the amount of light reaches the sensor or film. Suppose you want to create a misty effect on water surfaces (like those you see in so many nice pictures involving water), then you needs 4 seconds exposure. But in birhgt day light even if you close the aperture down to f32 you still would not get 4s, hence having a 8x ND will allow you to stop down 8x, and hence making long exposure possible in this situation. A closely related filter is ND Grad a filter that is clear on one end and gradually darken to the other end. This is used extensively for landscape photography when it is tricky to get the right exposure for the sky (much brighter) and the rest. The gradually darker portion will be adjusted so that that part gets reduced exposure but the rest will not be affected.
