What does a polarizing filter do and why do you need it?
A circular polarizing filter screws into the front of your lens. It has two layers of glass that when rotated will polarize light making colors deeper and removing reflections. You may already have a UV filter on your lens and for best results you should remove that before putting the polarizing filter on. The reality is you can screw a polarizer into a UV filter and stack them but you may wind up with a vignette effect (dark corners).
A polarizing filter is dark and you will lose two stops of light when using it so you generally would not use a polarizer inside. When using a polarizer outside you should consider the position of the sun in the sky. When the sun is higher in the sky polarization will appear more horizontal whereas later in the day the polarization will appear more vertical. The maximum degree of polarization happens at about 90 degrees from the position of the sun.
After you compose your shot you will rotate filter until you see the sky turn its maximum saturation. If you are looking to remove a reflection rotate the glass until the reflection goes away.
Many of you already have a polarizing filter kicking around your bag but may have never used it.
Remove reflections in glass, water and windows.
Tones down highlights so that they don't get blown out.
A neutral density filter reduces the amount of light coming into the camera allowing more saturated tones. These filters range from 0.3, 0.6 and 0.9 and also a variable version which will allow you to stop down up to 8 stops of light and slowing down shutter speeds. These are used for more dramatic effects like capturing moving water.
Capturing the image using these filters is not the same as post processing to achieve a similar effect. The end result will be far greater if the image was captured the best way. If you have these filters and have not been using them now is the time to start and see what you have been missing!