A white sauce can be any white sauce whether made with a roux, heavy cream, milk or half and half.  Traditionally, however, a white sauce is made by cooking flour with some butter to form a paste called a roux, then whisking in milk or cream until smooth.  Alfredo, on the other hand, is not thickened by a roux, but instead is made with butter, cream and plenty of Parmesan for the signature nutty flavor.