What is a double boiler?

A double boiler is essentially two fitted saucepans used as one unit, the smaller saucepan set inside the larger one. The larger one is kept partway full of water while you cook in the smaller one. Therefore, while it is heating, the hot water heats the above sauce pan, preventing scorching. It is used on the stovetop. The stove version of a double boiler is a bain-marie.