I'm sure alot of people have been wondering about the Halo 4 skins at what they actually are. They're is an amazingly big differ between armor skins and the armor premutations.
Armor and gun skins are used to customise the look of your armor peice, For example:
The skins are also currently being used for the guns as well as armor. There are many skins avaliable at depending on what retail store you buy it from. That doesn't mean that there won't be any amor skins you won't be able to get In-game.
For the list of retail armor skins, click here.