Sources of fat vary. Some recipes call for oil, some use butter, others use rendered meat fat.
using different fats will result in different types of Roux. A light Roux does not add much flavor to what it thickens but it thickens very well. A darker Roux will lend more flavor to a dish but will not thicken as well as a light Roux.
Darker Roux will be made using oil instead of butter. The longer it is cooked the darker it gets. The darker it is the more flavor it adds.
Step 1: Assemble your Ingredients
for a nice thick stew use a 1:1:1 ratio
for every cup of liquid you want to thicken combine 1 tablespoon of butter with 1 tablespoon of flour