The big difference is-
Skins are for 1 gun only, for instance an intervention skin would make every intervention look like that, while a CAMO will make EVERY gun with that camo look like that.
So skins are usually for customizing 1 gun, and they can change things like the magazine, scope, and other things that normal camo's don't change.
tl;dr.. Skins change the gun, camo's change the.. well, camo.