Polarising filters, in particular the circular polariser I have fitted to the front of my wide angle lens, are my favourite. They are a must for any landscape photographer and I have also seen them used for wildlife photography (something I am hoping to try on my next trip). Polarising filters work in the same way as polarising sunglasses work, allowing light of a particular single direction to pass through the filter. If you’ve used polarising sunglasses you will know what the effect is and that the colours seem to stand out more, blues are deeper and darker, greens jump out at you and other bright colours seem to almost be neon versions of their own appearance. This is particularly useful to a landscape photographer who wants to make the colours in his landscape really vibrant, especially the depth of a bright sky. Polarisers also reduce some of the light passing through the filter, requiring a one or two stop compensation on light metering compared no filter. This can help when a slower shutter speed is required to show movement in water for instance, however only a little bit.
Polarising filters usually come in the guise of circular polarisers which screws on to the camera lens but have two elements to the screw ring, one that is fixed to the lens and the other that is free to move, allowing the polarised filter glass to be rotated. This allows the filter to be adjusted to the light you are shooting in for the best polarising effect, however this needs to be judged by eye to avoid uneven darker and lighter areas of the photo. Using the circular polariser can therefore take some practice getting used to and I still end up with photos where the circular polariser was not rotated correctly. Lenses where the front element rotate when zooming and focusing (like the 18-55mm kit lens I think) can also be a problem as the polarising filter will need to be adjusted only after focus is achieved. One way to do this would be to focus, switch to manual focus and then adjust the filter. Or to shoot with manual focus from the beginning.
A note for polarising filters:
I was advised to always avoid using polarising filters when shooting people. The marks and blemishes on skin, when shot with a polarising filter, tend to be harshly exaggerated by the polarising filter in some instances. As I use my circular polariser on my wide angle lens, I tend to avoid shooting people’s portraits with it regardless to avoid distorting the image as a wide angle tends to do.