A roux is used as a thickener for gravy, other sauces, soups and stews. It is typically made from equal parts of flour and fat by weight. A traditional roux, used in most European cooking, consists of equal parts butter and flour. In Cajun cuisine, a roux is almost always made with oil instead of butter and cooked until dark brown in color, which lends more richness of flavor but less thickening power. The two different stages of a roux that are most commonly used are:
Blonde Roux: Used for soups and sauces that need to be thickened and kept a light color.
Brown Roux: Used in Cajun cooking or making sauces that need to be kept a dark color. A brown roux will also add a slight nutty flavor from the cooked flour