Roux- typically an even mixture of fat (butter or oil) and flour used to thicken a sauce or gravy. The fat basically lubricates and smooths the flour to prevent the forming of lumps when it is combined with a liquid such as milk for a gravy.
There are three types (classifications) of roux. White, blond, and brown. Each different roux is based upion the length of time it takes to cook. A white roux is cooked just until the flour and butter are evenly incorporated and should be removed from the heat before i begins to turn color (3-5 minutes). A blond roux is cooked a little longer, until it gives off a slight nutty aroma and turns an ivory color (5-7 minutes). The blond roux is comomnlly used in more creamy sauces or soups. Brown roux, largely used for Cajun or Creole cooking. Brown roux is cooked the longest (15-20 minutes and sometimes more)until it's dark brown and has a strong nutty fragrance. The longer you cook a roux, the less it will thicken a sauce or other mixture. Heat will eventually break down the starch in the flour.
Whether you're making a white, blond, or brown roux, let it cool momentarily before adding slowly whisking in your milk, stock, or other liquid. Once a roux and a liquid are combined, you need to whisk constantly until the sauce has thickened and come to a simmer. Use a low temperature to reduce the sauce to your desired consistency. Any trace of the "floury taste" should disappear after several minutes of slow simmering. If any lumps appear, strain your sauce in a fine mesh strainer/sieve before proceeding with your recipe.