Photographic filters are attachments that will allow certain kinds of light to be able to pass through, and reduce or even block out other unwanted kinds of light. Basically the term filter really means anything that changes the incoming light in some way. Common filters include: polarizing screens, multi-image prisms, close-up attachments, soft focus filters, neutral density (ND) filters, graduated ND filters, and color filters. Filters come in two main shapes. These shapes are round and square.
Filters can add color and certain looks to a picture that can complement the photographers personal style.
When you are using a black and white film with filters, you can darken or lighten the tone of the subjects color. The color filters that are used can darken the tone of an opposite color and brighten the tone of the color filter that is being used. For an example, a red filter will make the red leafs on an autumn tree look very rich and bright, while on the other hand the sky behind it that was blue, is now almost black.
Neutral density filters reduce the light entering your camera so that a slower shutter speed, or a larger aperture can be used. Graduated ND filters are used to balance out bright skies against darker foreground scenes.
There are hundreds of different filters to choose from. They are a useful tool in developing style and are definitely worth checking out.
Check out thesePhotographic Filters .