Essential oils are extracts of natural oils produced by plants - those which give a plant a fragrance. They can come from the leaves, flowers, stems, twigs, resin, fruit, roots or any other part of the plant. The oils are important to the plant for influencing growth, attracting insects for pollination, to protect from infectious agents or for repelling predators. They are a complex mix of bio-chemicals, which can have beneficial effects on the human body as well.
Producing even a small amount of essential oil takes a very large amount of plant material. For example, the oil extracted from the petals of 30 to 50 roses can go into a single drop of Rose oil (Rosa × damascena); that means that if there are 300 drops of essential oil in a 15mL bottle, about 15,000 roses were used to create the oil to fill that bottle. This demonstrates why some oils, such as rose oil, are so expensive!
One drop of essential oil therefore carries a very powerful concentration of the plant - this is why we only need up to a few drops to support the effects we're after.
It is important to that essential oils used for aromatherapy have high-grade purity. Fragrance oils are not appropriate – these oils are designed for producing a pleasing (although often synthetic) aroma and not for therapeutic effect.