Cornstarch is commonly used to thicken sauces. Commonly, anywhere from 1 teaspoon to 2 tablespoons of cornstarch are added to 1 cup of fluid. The mixture must be boiled to activate the cornstarch. When a recipe uses cornstarch in this way, you can often substitute one of xanthan gum, pectin, agar, or gelatine.
Cornstarch can be applied to wet items, drying them off so that batter will stick. This is commonly done for deep-fat frying. When a recipe uses cornstarch in this way, you can often substitute flour or cornmeal.
Cornstarch may be added to powdered sugar to prevent clumping.