Applying a fragrance oil or an essential oil?

Depends on what you're up to. I you want to use it for aromatherapy then go for the pure essential oils. This is so because a fragrance is a thoroughly tested combination of different synthetic and natural chemicals. These synthetics are usually created by aroma chemists in a laboratory facility. It is a way to try to chemically create a scent that is compatible with a scent of natural ingredients or plant extracts. Talking about plant extracts, is where essential oil come in. They are derived from parts of plants such as leaves, flowers, roots etc. These essential oils are used in aromatherapy and other processed in other products which need an aroma. Diffusers are used for aromatherapy and there's more about these diffusers at: http://www.beauty-styles.com/fragrance/best-essential-oil-diffuser.shtml