Essential oils are combinations of volatile chemicals that are produced by special glands in the plants they come from. They have many functions as I have written before ranging from attracting pollinating insects to deterring pests from eating them!
The chemical components include, ethers, alcohols, aldehydes, phenols,terpenes, ketones furocoumarins, acids to name just the main ones as well as numerous trace compounds. The furocoumarins are the compounds responsible for making Bergamot essential oil phototoxic meaning one should not go out in the sun or use a sun-bed for twelve hours after it’s use on the skin. This same group of compounds is also present in Giant Hog weed and Comfrey among other plants.
While I don’t remember all the compounds I learned once in organic chemistry at school I do remember that an ester is the product of the reaction between an acid and an alcohol.