Hi Redflare,
A "peep or aperture" sight is a type of iron sight mounted on the rear of a rifle. It is just a simple small hole that you look through, you then align the front sight on the target and fire. This sighting system is quite old but still very effective. It is found on many sporting, target and military rifles.
I like to use a peep sight for a few reasons. You look through the peep hole but not at it, this allows you to focus on only 2 things, the front sight and the target, instead of 3 things. I find this is getting more important as I reach middle-age and my eyes are not focusing as quickly as they once did. I usually hunt with a scoped rifle (magnification, light gathering ability and single focus plane are an asset to me) but the peep is fast and effective at close range. A rifle with iron sights is more handy, can be more rugged and I find it easier to keep clear in foul weather (although quality scope caps help a lot).
I am going to put the peep sight on the .22 Marlin Papoose just to keep it light and handy, I rarely shoot at game over 30 yards with this rifle.
I found a good description of Iron Sight Types at Wikipedia located here
http://en.wikipedia.org/wiki/Iron_sightsTalking guns is fun,
Mike