A roux is a mixture of flour and fat, that after being slowly cooked over low hear, is used to thicken mixtures such as soups and sauces. There are 3 classic roux: white, blond and brown (dark). The color and flavor is determined by the length of time they are cooked. White and blond roux are made with butter and traditionally flavor white sauces or light soups. Dark roux usually also contain pork or beef drippings or lard and are cooked to a dark color (sometimes for up to an hour) and used to thicken rich sauces, and soups like gumbo.