"Ghost ring" has been defined adequately. All I wanted to add is that once you try one, you'll never go back to standard sights. I've had the SGT Traklok (sp) on two different 870's and absolutely loved it - enabled me to shoot those guns like a rifle out to about 100 yards.
Now I have a Ghost Ring on my Marlin GG (Ashley Outdoors), and I feel that I can hit anything I can see.
"Happiness is a warm gun" John Lennon 1966