Inverted sugar syrup is sucrose-based syrup treated with the enzyme invertase, which splits each sucrose molecule into one glucose and one fructose molecule, giving a more well-rounded sweetness and preventing crystallization. It is marketed under various names, including Golden Syrup (Tate and Lyle). It is also used in fondant sweets.
Partially inverted sugar syrup can be home-made without the use of enzymes: During the production of simple sugar syrup (see there), one adds about one gram of acid, such as citric acid or ascorbic acid, per kilogram of sugar. In addition the boiling time is extended to about 20 minutes. This preparation will split a good part of the sucrose molecules and effectively prevents crystallization, without giving a noticably sour taste.
External link