The most common method of extraction is through the use of water and/or steam. There are different ways to go about extracting oil, and in most of them water is heated to produce steam which carries the most volatile parts of the material with it. The steam is cooled in a variety of ways producing condensation which is collected. Typically the Essential Oil will float to the top and the remaining water is called a Hydrosol.
True Steam distillation uses an outside source of steam which pipes the steam into the distillation unit, sometimes at high pressure. The steam passes through the aromatic material, and exits into the condenser. Another method is to submerge the materials in water completely as in making a soup and is possibly the best method for very dense and rough materials like woods, nuts and roots. Additionally steam and water distillation can be achieved by steaming material over boiling water in something similar to a wok or basket, a method suitable for softer materials like leaves.