A filter is a special optical aid that can be affixed or mounted in front of the lens of a SLR camera, or can be placed in a special holder accessory.
There are various reasons for using a filter. For example a technical filter can be used to remove the blur and haziness of a photograph created by the Ultraviolet (UV) light produced by the sun. Or a artistic filter is often used for removing the reflection while photographing a fish in water.
For more information about which filter to use for what purpose, please click here to find out more.