In short, not all frostings are buttercream, but all buttercreams are frosting. Frostings can be any type of glaze, syrup, decoration put on top of a cake. It can be a simple mix of powdered sugar and a splash of milk, or made richer with melted butter, icing sugar and milk. A frosting can be anything. However, buttercream is a whipped mixture of plenty of butter, powdered sugar, flavoring, and a splash of something liquid to bind it all together.