A Bain Marie or double boiler is a pan placed on top of another pan. In the bottom pan, hot water simmers. In the top pan, whatever you are making is stirred.
Double boilers are necessary when the food should not be warmed above a certain temperature, or it will curdle. This is especially true for sauces and puddings with egg yolks and some candies.
My favorite double boiler is this one from WMF, a German firm. It has a rounded bottom so the sauce is stirred evenly.
