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

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 the containers to rise as needed. The constant boiling temperature of the water helps to keep contents of the inner pot from boiling or scorching.