Roux - equal amounts of flour and fat are combined and cooked. Cooking the roux eliminates the raw starchy taste often associated with flour. There are three kinds of roux: white, blonde, and brown. Examples of recipes where the roux would be used would be: white sauce or bechamel, cheese sauce, cream soups.
Cornstarch - is a product made from corn. It is tasteless and is used as a substitute for flour as a thickening agent. Because it is a "superpower" for thickening not as much is required for thickening. To prevent lumping cornstarch and cold water should first be mixed into a paste. It can then be added to the product requiring thickening. Sweet and sour sauce, lemon sauce for chicken and almond sauce are all examples of a recipe where cornstarch is used. Cornstarch is also sometimes used in making gravies.
Whitewash - a thin mixture made by combining flour and cold
water. It is also known as "a slurry". It is inferior in flavor and
texture to a roux. Not recommended by chefs for sauces. Whitewash is
used in the production of gravies and stews.