What is a "roux" and what are its uses?
It's actually spelled 'roux', and is a mixture of oil and flour, cooked to remove the starchy taste of the flour.

It's a great thickener any time that you don't need the sauce to be clear, and you have time to cook it down.  I typically use it for cream sauce (including cheese sauces, such as for mac &amp; cheese) and gravies.

As for benefits -- it's habit at this point, so I'm not really sure -- I guess it's made from things I always have on hand.  You can also get flavor from the roux, if you cook it longer , but you'll adversely affect the thickening ability.  Gumbo is normally made from a dark roux (the Cajuns have a series of names for the color of roux, including 'brick', 'peanut-butter' and 'chocolate')