Corn starch is used as a thickening agent in soups and liquid-based foods, such as sauces, gravies and custards by mixing it with a cold liquid to form a paste or slurry. It is sometimes preferred over flour because it forms a translucent mixture, rather than an opaque one. As the starch is heated, the molecular chains unravel, allowing them to collide with other starch chains to form a mesh, thickening the liquid (Starch gelatinization).
It is usually included as an anticaking agent in powdered sugar (10X or confectioner's sugar). For this reason, recipes calling for powdered sugar often call for at least light cooking to remove the raw corn starch taste. Baby powder often uses cornstarch.
Corn starch when mixed with a fluid can make a non-Newtonian fluid, e.g. adding water makes Oobleck and adding oil makes an Electrorheological fluid.
A common substitute is arrowroot, which replaces corn starch on a 1:1 ratio.