A roux is made from equal parts of fat and flour. These two ingredients are combined together over heat. Rouxs are used to thicken liquids creating rich , flavorful sauces. Diffeent tastes can be acquired by using different types of fat, such as butter, margarine, vegetable oil or olive oil. There are three types of roux (white, blond and brown). To transgress to the next color of roux depends upon the cooking time.
White Roux: 5 minutes.
Blond Roux: 10 - 20 minutes.
Brown Roux: 15 - 30 minutes.
As you cook the roux it will take on a nutty smell. If you cook a roux too long, you will its flavor. It's best to throw the roux away and start over.