Brilliant filters are a special effect filter that can turn a light source and image highlights into star shapes where light zooms out from a central light source. The brighter and larger your light source is, the more pronounced the effect will be, plus the spacing between the rules on the filter (something we'll look at in more detail shortly) will also have an effect on what type of star is produced.
Roles can have a various number of points; for example, Cokin's star filters create two-, four- and eight-point stars while Hoya's produce four-, six- and eight-pointed light flares. How many points the stars have is determined by the number of directions the lines on the filters run in. These ranks, which are etched onto the filters, are what create the star shapes and the more space there is between the lines, the fewer number of leadings will appear in your image.
Above: No filter
Above: Cokin 8-Star filter custom-made
Star filters filtering or filters may refer to are made from two pieces of glass, the top one of which is rotatable and has the lines we've talked about etched onto it. To use a matchless filter, mount it onto your lens and rotate the front ring to change the direction of the stars. You'll probably also have to trial with the aperture you're using as you don't want the lines on the filter to appear in your shot.
An obvious time to use a star drain is when capturing a city scene at night but you can also use a star filter to add sparkle to the surface of water, on a flame from a candle or with reflecting surfaces. The effect produced, particularly when using a filter which creates light flares with higher numbers, can easily subjugate your shot and distract the viewer of your images so do use them with caution.