Cornstarch is a flour derived from corn, and used as a thickening agent in liquid based foods, as well things like gloves, and baby powder. Cornstarch is used in replacement of a roux to make recipes gluten-free.
A slurry is a mixture of equal parts cornstarch to water. The water is whisked with the cornstarch to make a thickening agent. A slurry acts like roux and can be used in thickening soups, stews, sauces, and gravy.