A double boiler is a double-walled cooking pot with water in between the walls. This construction limits the maximum temperature to the boiling point of water, which is generally about 200 °F to 212 °F (93°C to 100 °C). Two suitably sized pots can function as a double boiler, provided that the inner pot can be kept from touching the outer pot (except perhaps at the top edge) and can be kept stable enough.
If the inner pot seals fairly well against the outer pot, it is not necessary for the inner pot to touch the water.
Double boilers are most commonly used to melt chocolate. They can be used to cook custard, preventing the outside from crusting before the inside is fully cooked. Double boilers are also used in the making of Hollandaise sauce and custard-style ice creams to cook egg yolks.