Essential oils have been making headway in their popularity over the past decade. Multiple uses exist and can provide a better alternative for those who react to synthetic products or are just looking for a healthier alternative. The following link is to a blog post of Young Living that provides some basic answers to Essential Oils, as well as some of their products.
Now, as I am a distributer for Young Living, I do tend to promote them first. However, doTerra is another brand that seems to also provide a high quality product. There are other brands in the market, but do research before investing. As mentioned in the above blog link, other brands may use preservatives or synthetic fillers to help the shelf life of their product, or to even dilute the oil you are purchasing. As my research has shown, what you pay for is what you get.
Also, always research an essential oil before you put it on your skin or ingest it! Some oils are harmful if you eat them, as well as irritate the skin if the oil has direct contact. Sometimes, a carrier oil is all that is needed to allow the essential oil to provide benefit and not a rash!