AW - it's hard to describe what a polarizer is exactly, but it's a filter that many landscape photographer's use to 1) make the colors more saturated esp blue skies and 2) reduce or remove reflections in windows, water, and other stuff. You rotate them to increase or decrease the polarization effect. They increases the richness of your image and perhaps the contrast too (sometimes), but also make the images look different - usually for the better, but not always - they can make some shots look unreal, esp shots of people. Also note that they block a significant amount of light - you will lose between ~ 0.5 to 2 f-stops of light. Almost all landscape photographers carry one with them.. They're the # 1 most popular filter.

G