A bain-marie (also known as a water bath or double boiler), a type of heated bath, is a piece of equipment used in science, industry, and cooking to heat materials gently or to keep materials warm over a period of time.

The name comes from the medieval-Latin term 'balneum Mariae', literally, Mary's bath, from which the French 'bain de Marie', or 'bain-marie', is derived. The device's invention has been popularly attributed to Mary the Jewess, an ancient alchemist.

The double boiler comes in a wide variety of shapes, sizes, and types, but traditionally is a wide metal container made of three or four basic parts; a handle, an outer container that holds the working fluid, an inner, smaller container that fits inside the outer one and which holds the material to be heated or cooked.

Typically the inner container is immersed about halfway into the working fluid. The smaller container, filled with the substance to be heated, fits inside the outer container filled with the working fluid (usually water), and the whole is heated at, or below, the base, causing the temperature of the materials in both containers to rise as needed.

More Info: en.wikipedia.org