Royal icing is a hard white icing, made from egg white and icing sugar (mix or pure). A little addition of an acidic substance like lemon juice, citric acid or cream of tartar, the texture of the icing can be improved. I use white vinegar and am very happy with it. Royal icing can be used on products like Christmas cakes, wedding cakes, gingerbread houses and many other cakes and biscuits, either as a spreadable smooth coating or as piping decorations. Royal icing is also used to make decorations such as flowers and figurines for putting on the cake.
One way to use royal icing is by piping ornaments or decorations in certain shapes or forms over a plastic sheet. When the decorations are completely dry (overnight is recommended) they can be lifted and glued with soft royal icing onto a cake.
Additional added liquid like lemon juice, water or egg white will bring royal icing into a runny condition to coat products like cookies or dry baked pastries. Flooding work is commonly used with royal icing.