There are many filters that can be put on the end of a camera for special effects. One that creates a unique effect is called a star-effects filter. For an SLR camera, digital or film, the lens is threaded at the end. A filter will screw into the end of the lens. In the case of a star-effects filter, the filter can be turned on the end of the lens once it has been screwed in. What it does is any point of light the camera is pointed at, such as a streetlight or the sun or the moon, the filter will create the effect of steaks of light coming off of that light, creating a "star" effect. If used properly, it can add a dramatic and powerful effect to an image. It will cause the image to stand out and take the viewer's attention immediately. One instance that I used the star effect filter was on a closeup of our Christmas tree at home, creating a radient image. The filter can be turned in order to position the streaks of light perfectly for the composition.
In order to show what is meant by a star effects filter, here are two images that have become favorites of mine. I took the same pictures without the filter and they are not worth a second look.