Essential Oils are concentrated liquid aroma compounds from plants. An oil is "essential" in the sense that it contains the essence of the plants fragrance. They have been used medicinally in history for curative purposes.
The most common method of extracting essential oils is distillation. You boil the leaves or flowers of a plant and use a distiller to separate the oil from the water of the plant.
Citrus oils can be mechanically expressed or cold-pressed.
Flowers generally contain too little oil to be expressed, but are too delicate to be distilled. So a solvent such as supercritical carbon dioxide us used to extract the oils.
Essential Oils have been known for centuries for their medicinal properties. The oils have specific chemical compounds, such as methyl salicylate or "oil of wintergreen." The medical community tends to see the use of essential oils as "Pseudoscience" in terms of healthcare, however, the DO contain chemicals which can provide the same results as Man Made Drugs.
Some COMMON essential oils you may have heard of are Menthol, Capsaicin, Anise and Camphor. These are used in "Icy Hot," Vapor Rub and Capsaicin (the active ingredient in Hot Peppers) is in ground Pepper.
Many essential oils affect skin and mucous membranes in ways that are Harmful or Valuable. Tea Tree oil may cause Contact Dermititis. It is used in Antiseptics and Liniments. They produce irritation at first and then numbness. Turpentine oil and camphor are two examples of this. menthol and some others produce feelings of cold followed by burning. Some oils, such as clove oil, were used for dentistry as antiseptics and local anaesthetics.
Aromatherapy is a form of alternative medicine in which healing effects are felt from the essential oils which are mixed with other "carrier" oils, such as jojoba. These are thought to have effects such as relaxation.
Herbs, Roots, and Flowers make up essential oils. Common oils we use can be found here.