Does the difference lie in ingredients and/or technique?
Icing is what happens to the road on a cold winter day. Frosting is what happens to your tomato plants on a cold spring day.
Frosting can also happen when you receive a smart aleck answer to an honest question so…
Culinarily speaking, there is no difference, just different words that describe the same thing.
In my kitchen, icing is basically another word for glaze. It might include flavors (such as lemon juice or Grand Marnier), spices (such as nutmeg), or a small amount of butter, but it's the combination of powdered sugar and a liquid. The texture is smooth and the consistency is thin enough to drip down the sides of a cake.
Frosting is what I call any sweet substance that's thick enough to require spreading or smoothing or piping onto a cake or cookies. In my kitchen, that can mean anything from a mixture of butter, powdered sugar, and cream cheese, to pureed cashews and blackberries or whipped coconut cream (vegan versions of frosting).
I suppose I use the terms mainly to distinguish between consistency and technique, though ingredients always influence both of those. Such a fascinating question; I can't wait to hear how other people distinguish them!
I'm not sure, but in my experience the usage seems fairly interchangeable. Maybe it's regional? Like pop : soda, or hoagie : sub (btw, the correct terms are soda and sub. Just saying.)
I guess when I think of icing, I think more of a coating that hardens, like a black & white cookie, and frosting as something that remains soft and creamy, like buttercream, etc. Technically, that may not be correct - but without lifelines, that's my final answer.
Here is a whole article on the subject. It supports Beyondcelery's response, and also factors glazes into the mix: http://www.cakespy.com/blog/2009/1/14/the-icing-on-the-cake-an-exploration-of-icing-frosting-and-e.html