Royal icing is a thick paste made from egg whites and sifted icing sugar. It is spread over marzipan-covered fruit cakes, or piped into decorations, and left to dry out. It is mainly used on rich, highly decorated celebration cakes.
Glace icing is a smooth paste of icing sugar dissolved in a little water, which is spread straight onto the cake and left to dry out. It is ideal for covering cup cakes, tea breads and simple sponges. Ready-made and fondant icing are the same thing: icing sugar, egg whites and liquid glucose are mixed and kneaded to a soft dough which is rolled out before being smoothed over a cake. Use it on simple cakes, novelty and celebration cakes.