Cornstarch has multiple household purposes. It can be used in cooking as a thickener for sauces or soups. It can be used for window cleaning (link to instructions below).
When thickening a sauce with cornstarch dissolve the cornstarch in a small amount of cool liquid before adding it to the larger amount of warm or hot liquid. Cornstarch does not mix into hot liquids well and is likely to create lumps. When the cornstarch is first added, it will be cloudy. As it thickens the sauce, it will become clear. It should be stirred periodically as it heats and thickens, but too much stirring will thin the sauce.
Some recipes have food that's coated in cornstarch before it's cooked -- for example in Chinese dishes cubes of chicken are often coated before stir frying, similar to flour coating for chicken before it's fried.
- More cornstarch tips from FoodReference.com.