An essential oil is a product that is extracted in one way or another from natural material such as leaves, flowers, bark or fruit to name but a few. They each have a variety of properties ranging from antiseptic to anti-inflammatory.
When looking for an essential oil you should make sure that you buy one that states pure essential oil on the label. Many oils available in the shops are blended in a carrier oil which means that there is a small amount of pure essential oil mixed in a high percentage of oil such as jojoba. These oils offer poor value for money as they are often priced on a par with pure oils and are in my opinion a bit of a con.