Essential oils are produced through a process of distillation with steam or water. They are the largest category of fragrance materials and the most widely available.
Some perfume molecules decompose under the heat of distillation. In this case oils are separated from fresh flowers by solvent extraction. A liquid solvent is circulated over the flowers to dissolve the essential oils. The process yields a concrete which is semisolid.
By dissolving a concrete it becomes an absolute, a highly concentrated liquid essence. Absolutes are floral essences at their truest and most concentrated. They are more long lasting than essential oils. Absolute is the most expensive perfumery ingredient.
Carbon dioxide extraction is similar to distillation. With enough pressure carbon dioxide is turned into a liquid that acts as other liquid solvents do. It is pumped into a chamber filled with plant matter to extract the essential oil along with other substances such as resin.