There is a big difference between frosting and icing. Frosting is thick and holds shapes like rosettes and shells like those you see piped around the edges of a birthday cake. It remains soft to the touch and has a creamy texture, and most people think it tastes better because of the creamy buttery flavor. Icing, on the other hand, is a thinner, more liquid substance, and as it dries it thins out, becomes very smooth across the surface of your cookie, and hardens. This is the icing to use for the most beautiful, professional results.
Ah! I had a feeling that question would come up! I actually changed my last comment from "frosting" to "icing" because I use both words interchangeably and couldn't decide which one I wanted. Darn English.