A syrup is the way of adding flavor or sweetness to a drink at Starbucks. There is a variety of syrups from vanilla to peach blossom.
A sauce is similar but used for multiple purposes. A sauce can be used to add flavor to a latte or Frappuccino, but it can also be used as decoration, or drizzle as a topping for whipped cream on a drink when stored in squeeze bottles. It is thicker than a syrup and contains corn syrup rather than cane sugar.
In cooking, a syrup or sirup (from Arabic: شراب; sharāb, beverage, wine, via Latin: sirupus) is a thick, viscous liquid consisting primarily of a solution of sugar in water, containing a large amount of dissolved sugars but showing little tendency to deposit crystals. The viscosity arises from the multiple hydrogen bonds between the dissolved sugar, which has many hydroxyl(OH) groups, and the water. Syrups can be made by dissolving sugar in water or by reducing naturally sweet juices such ascane juice, sorghum juice, or maple sap. Corn syrup is made from corn starch using an enzymatic process that converts it to sugars. Technically and scientifically, the term syrup is also employed to denote viscous, generally residual, liquids, containing substances other than sugars in solution.