By definition, essential oils are natural oils that are highly concentrated from the source (can be shrubs, flowers, trees, roots, bushes and seeds) that it's extracted from and are also known as volatile oils. They are usually extracted through steam distillation, but can also be extracted by cold pressing. Essential oils have been around for many years due to their many uses: flavouring, adding scent and health uses. You can either dilute the specific oil in a carrier oil which is then massaged into the skin, diffuse the oil in the air, heat over a flame or burn it as incense.
Essential oils as well as plant extracts have been known throughout history since the beginning of time and are considered by many to be the missing link in modern medicine.