There are many different filters for a camera that achieve various effects. One of my favorites is a circular polarizing filter. It screws onto the end of my lens and is relatively easy to use.
Its purpose is to make colors more vibrant, particularly greens and blues. I use it a lot when shooting water scenes because it reduces reflections and intensifies colors. Reducing reflections is the big advantage. In post-processing editing software, it is possible to achieve a similar effect in intensifying colors without using a circular polarizing filter, but the water and reflections won’t look the same.
The only disadvantage I’ve come across is the amount of light coming into your lens is greatly reduced, almost by 2 stops. Therefore during dawn and dusk, you should use a tripod, or adjust ISO, speed, or aperture to make up for lost lighting, or skip using it. Shooting also works best if the sun is to either side of you, otherwise the sky may seem unevenly blue. The filter has two strengths – if you turn the filter a quarter turn while looking through the viewfinder, you’ll see a difference.
Tip: Instead of purchasing a new polarizing filter for each lens you own (if they don’t have the same circumference), you can save hundred of dollars by simply buying a step-down ring (costs $10-$20) that allows a larger polarizing filter to fit on a smaller lens by first attaching it to this step-down ring.
It takes a while to get the hang of anything new, but the results can be very effective.