Get out.
They are DIFFERENT. And those of you who have had homemade versions of them would KNOW THAT. JERKS.
Frosting is more common. It is mounded and spread on cakes. It's thick and stiff (that's what she said) and it stays in place. It maintains consistency and forms well.
Icing is less common (except on cookies.) It starts runny and ends kind of crisp and glossy. You can't spread it because it just goes everywhere.
A good example. You use frosting for layer cakes. You use icing for sticky buns and bundt cakes.