Roux is a cooked flour-butter mixture used to thicken many sauces. It is made by whisking flour into equal amount of melted or clarified butter, then cooking the mixture until a desired colour is achieved: white, blond or brown.
White roux roux blanc is cooked just long enough to lose its floury taste. It is used to thicken white sauces.
Blond roux roux blond is cooked until golden in colour. It is used to thicken white sauces.
Brown roux roux brun is cooked until light brown in colour. It is used to thicken brown sauces.