I have read that royal icing becomes really hard on drying. Is it hard candy like?
I have also read that royal icing stays soft from inside and crusty on the outside (for writing piping) when corn syrup is added to the recipe.

The other day I saw a video that said that they used royal icing for icing a cake.
I wanted to know whether it is a good idea to ice a cake with royal icing and does it taste good? I thought that royal icing was just used to ice cookies.

that really depends how much sugar you add.  In Britain,  they normally use royal icing to ice their cakes.  Like all icing,  it is sweet.  Taste like meringue just more firm.

ok! Thanks Pretty cake! what about the American royal icing? the one on cookies.. does it go really hard? Will people with weak teeth or dentures be able to eat it? does it dry out like gum paste?

American royal icing like the one on cookies does not dry out nearly as hard as gum paste or hard candy. You can easily break off the cookies by hand. So yes people with weak teeth or dentures should be able to eat cookies with royal icing decoration.

Thanks a lot Jenn, for clearing that out!