Cornstarch is a fine, powdery starch that is made out of corn. The cornstarch is actually made from the endosperm of the corn, which makes up most of the kernels that we eat when enjoying popcorn or corn on the cob. Cornstarch, also sometimes called cornflour, is produced by grinding, washing and drying the endosperm of the corn until it reaches that fine, powdery state. Cornstarch is gluten free.
Cornstarch has many culinary uses, but it is most often used as a thickener for sauces, gravies and fruit pie fillings. Cornstarch thickens very quickly and easily, and forms a clear sauce after cooking, rather than an opaque one. It has roughly twice the thickening power of flour, and while it is flavorless after cooking, it does need to be cooked for a short period to remove any starchy flavor from the starch, as well as to give the mixture it is used in a chance to thicken. Unlike flour, cornstarch will clump up if added directly to hot liquids and must be mixed with a small amount of color liquid before being incorporated into something hot, such as a gravy or a pudding. It if is added to a cold mixture, it does not need to be prepared in any way before cooking and will dissolve as the mixture heats up. If cooked for an extended period of time, or whisked too vigorously, a mixture thickened with cornstarch can break. Arrowroot and tapioca are both good substitutes for cornstarch when it comes to thickening power.
Cornstarch is also included in many baked good recipes, and is often used in conjunction with flour. Since it is gluten free, cornstarch can help add some structure to a baked good while increasing its tenderness. It appears very often in shortbread recipes, where bakers are looking for a very crumbly and tender texture in the finished product. Another common way of using it is adding a small amount of it to all purpose flour to make a substitute for cake flour. You will also often see it included in batters, where it helps contribute to a light crust after frying.