Specifically it is a cooked mixture of flour and fat. The color varies from blanc de blanc to dark chocolate and is directly related to the time and temperature the roux is cooked at. The thickening power is inversely proportional to the darkness of the roux.
You will roux the day you miss my class in sauce making.
WWG1WGA
Garde la Foi, mes amis! Nous nous sommes les sauveurs de la République! Maintenant et Toujours!
(Keep the Faith, my friends! We are the saviors of the Republic! Now and Forever!)
LonePalm, le Républicain du verre cassé (The Broken Glass Republican)