Ghost rings are basically a faster pointing cousin of the traditional aperture, or "peep", sight. That is to say a ring for the rear sight and either a blade or a post for the front. The human eye will naturally try to center the tip of the blade or the post in the ring thus creating the sight picture.
The ring of a ghost ring is larger that the opening of a pep sight as the goal of a ghost ring set-up is quick target acquisition and this is easier with a bigger rear opening.
When using ghost ring sights, you look through the ring (ignoring it, basically) and focus on the front sight; just like traditional rifle and pistol sights. When you do this the rear sight is quite out of focus and sort of fades form your vision, hence the name "Ghost Ring". Your eye and brain, however, will still register the ring and will naturally try to center the tip of the front blade or post in the ring.
Ghost ring sights are great for shotgunning. While not quite as precise as the smaller aperture on a peep sight, the extra precision is not really missed for most shotgunning applications. The quick target acquisition and sight picture are usually of more value for tactical shotgunning or even taking out fast moving small game.
While there are many different ghost ring sights out there, I doubt that you have actually seen a ring on both the rear and front sights... I'm guessing that what you saw was a front blade that was protected by wings on either sides, such as the LPA sights on a Mossberg. But then again, who knows... maybe some manufacturer somewhere is trying something new.
As for the many different configurations and brands; the two best known (and best to stick with) are Scattergun Technologies (made by Wilson Combat) and the Italian made LPA sights. Both are excellent and both are sold and installed by Tactical Ordnance (
www.tacord.com). Give Casey a call or shoot him and e-mail and he can recommend which ghost rings he thinks are best for your particular gun. FWIW, the LPAs can be had with fiber optic inserts, which are very nice, but the Wilsons can be bought with tritium inserts... So there are lots of choices...
LPA does make a rear sight that will mount on a rail. This is great if you have a gun with a top rail and don't want to permanently mount a sight behind it. But if your gun doesn't have a rail, then you are better off just getting the rear sight installed properly. Hell, even if you do have a top rail, you might as well just get the rear sight properly installed and save the rail for an Aimpoint Micro someday down the road...
The one thing to keep in mind though is that installing the sights will involve soldering for the front and drilling or soldering for the rear, depending on which brand you go with. I guess what I'm hinting at is that you should probably get the sights installed first and then get the gun re-finished... So if you really do want ghost ring sights, you should decide quick before Murdoc starts his work on your gun.
Hope this helps. If you have any more questions, just ask.
Cheers!