Most white sauces, in all their incarnations, are based on a roux (a flour-based paste). White sauces vary in what you add to the roux. The most common type of white sauce is a béchamel sauce (pronounced besh-ah-MEL).
Béchamel sauce, with its buttery, faintly nutty flavor, is the base of hot soufflés and such homey dishes as macaroni and cheese and pot pies.
You can modify béchamel in many ways to suit the dish it garnishes. For example, if you’re cooking fish, you can add fish stock to the sauce. If you’re cooking poultry, you can add chicken stock.
A velouté (pronounced va-loo-TAY) is essentially a béchamel made with a stock (fish or chicken) in place of the milk, which gives it extra flavor. Sometimes, you enhance a velouté before serving by adding a little cream (for a smoother texture) or some fresh lemon juice (for a little tartness).