Essential oils are simply concentrated plant essences.
Although they are called “oils,” this is something of a misnomer, because most essential oils are not in fact oily, unlike vegetable oils which have been expressed from seeds or plant nuts such as sesame, sunflower, peach kernel, or sweet almond.
Essential oils are aromatic substances that derive naturally from plants. For example, rose oil is made from roses, eucalyptus oil from eucalyptus.
The oils affect the human body in two ways: through the nose and through the skin.
Aromas disperse in the air, float, and eventually reach the nose. The aromatic molecules float up the nostrils and come into contact with nerves extending from the olfactory bulbs, and ending in two small, sticky patches at the top of the nasal cavity.
When the aroma molecule hits receptors in these nerve-rich patches, it causes healthy brain activity.
Essential oils do more than just smell good: they have varied and documented benefits when applied to the skin. Nature makes antibacterial essential oils such as Tea Tree, Lemon, and Geranium, as well as anti-inflammatory and antimicrobial oils such as Bergamot, Lavender, Eucalyptus, and Thyme.
Some oils are applied directly onto the skin, neat, but usually it is better to dilute them in a natural plant-based carrier oil (such as sweet almond oil), as I do in my products.