If you're going to hunt with slugs, rifle sights are almost necessary.
There are two basic types:
The standard type are the barrel-mounted front and rear.
The rear sight is a simple notch-type.
These work well for most people, and are less trouble to get, since you usually just buy a slug barrel with the sights already mounted.
The so-called Ghost ring is simply a new name for the peep, or aperture sight as used on most military firearms.
The peep is almost certainly the best, fastest to use and most accurate sight.
To use, you simply look THROUGH the rear aperture and focus your eye on the front sight.
Since your eye automatically centers the front sight in the aperture, you aren't trying to focus on both the front and rear sight and trying to align them.
Installation of ghost ring sights is a little more complicated.
The front sight must be brazed onto the barrel, in most cases, and the rear assembly is attached to the receiver by drilling and tapping two holes.
This is usually best done by a gunsmith with experience at doing the job.
Due to the red heat needed to braze the front sight on, the barrel needs to be refinished.
As an interesting option, there is a new front sight available that clamps to the barrel. This requires no gunsmithing or alterations, and greatly simplifies the job.
In addition, you can remove the sights easily if you desire.
http://www.brownells.com/aspx/NS/store/ProductDetail.aspx?p=9203&title=TACTICAL+SHOTGUN+SIGHTS
A great many ghost ring users are buying the Wilson sight, and this is the sight Remington is using on their Police guns.