With a peep sight the rear sight opening is much smaller and you sight through the rear sight to the front sight.
With a ghost ring the rear sight opening is much bigger, you only look at the front sight. The rear sight basically "blurs" or is not seen (ghost) by the eye but the rear sight automatically centers the front sight in the "ghost" ring. It is an optical thing. Pretty cool actually, you really have to try one to see how it works.
As far as I know some peep sights have a removable center piece/aperture which would increase the size of the opening of the rear sight, maybe that's how these are.
