It is the chemistry of an essential oil that gives it the therapeutic properties.
Essential oils contain hundreds of different chemical compounds, which in combination, lend important therapeutic properties to the oil.
Although chemists have successfully recreated the main constituents and fragrances of some essential oils in the laboratory, these synthetic oils lack therapeutic benefits and may even carry risks. Also, many essential oils contain molecules and isomers that are impossible to manufacture in the lab. This is one of the main reason why essential oils cannot be classified as "drugs".
Because essential oils are composites of hundreds of different chemicals, they can exert many different effects on the body. For example, clove oil can be simultaneously antiseptic and anaesthetic when applied topically. It can also be antitumoral. Lavender oil, often called the "Swiss Army Knife" of aromatherapy, is beneficial for various uses such as burns, insect bites, headaches, PMS, insomnia, stress, and hair growth.
One very important thing to note is that because of their complexity, essential oils do not disturb the body's natural balance or homeostasis. If one chemical constituent exerts too strong an effect, another constituent may block or counteract it. A great example of the intelligent molecules of plants working in harmonious synergy within the human body to enable healing!
Synthetic chemicals, in contrast, usually have only one action and often disrupt the body's own balance. So, we begin to understand that not all "chemicals" are created equal!