Skin, referring to liquids like milk, is a film that can form across the surface, especially when milk is boiled. The skin can be skimmed (remove floating matter from a liquid) off the milk. I had not heard of "skim" as a noun before, but my dictionary says it can refer to the thin layer or film that is skimmed off the milk. So it sounds like 2 words that can mean the same thing. "Skim" is most often a verb, and as a noun being something that has been skimmed. "skim milk" is milk which has had the cream removed from it.