a roux is a thickening base for a lot of sauce and soup recipes, and consists of 1 part fat/1 part plain white flour.
there are 3 classifications of roux, they are:
white roux - as for bechamel
blond roux - as for veloute's and tomato soup/sauces
brown roux - as for espagnole
method, basically melt the fat, add the flour and mix over a low/moderate heat and cook to a sandy texture without colour.
if making a sauce add the stock (hot) gradually a ladle at a time mixing in gradually, this will ensure that the sauce will not go lumpy.
when i make a soup with a roux i add white white to the roux and mix before i add the the stock.
alternatively if you wanted to thicken a puree soup or veloute sauce you can use what is called a 'beurre manie', which is equal quantities of melted fat and flour mixed together and then blitzed through the hot sauce/soup to thicken it.
types of sauces that do not require a roux are mainly egg or cream based.
hope this information helps
