Classic Roux is made by blending equal parts of butter to flour.
There are three stages of roux:
White, Almond & Brown. Each one gets its flavor and color by cooking the flour-butter. The Darker the roux, the more rustic and nuttier the flavor.
You can use it for Gumbo, Acer, but I don't recommend it. Gumbos themselves can get pretty thick -especially if cooking them with okra, beans or tomato paste.
Roux is used to make four of the five mother sauces -excluding "Naise" Sauce or hollandaisse.
I am actually going to post a shellfish gumbo for the recipe week!