Invert sugar is the combination and heating of sugar syrup with a small amount of acid, e.g. cream of tartar or lemon juice, which will "invert" or break down the sucrose into its components, glucose and fructose. Because this creates very fine sugar crystals, invert sugar is used in making candies and fondant which require a smoother product. The process of making jams and jellies automatically produces invert sugar by combining the sugar with the acid in the fruit.
Invert sugar can be found in jars in cake-decorating supply shops.