I have been using essential oils for years for aromatherapy, making crafts such as soap and bath salts, and various other uses, and I always bought them in the grocery store. However, I did not know that there was a big difference between using the scents of plants and the actual plant oils until a friend introduced me to Young Living essential oils. The Young Living essential oils are (usually) steam-distilled from plants grown on their own farms with organic methods, and all their oils are tested by independent laboratories to ensure that the oils are pure and contain a high enough percentage of the correct molecules to be effective.
I attended a class on the chemistry of essential oils to learn more, and discovered how the molecules of the oils interact with the molecules of your body, and from then on I was sold! It turns out that the oils you buy in the grocery store contain one or two of the molecules found in the plants, and sometimes those molecules are reversed (left-handed instead of right-handed, or vice versa), and not only that, but I could actually learn to smell the difference between the left-handed and right-handed molecules. In addition, I learned to smell the difference between monoterpenes, sesquiterpenes, and diterpenes and to be able to discern the same molecule in such different oils as orange peel oil and black pepper oil.
Since that class, I have smelled many perfume grade and other oils, and the difference between the oils you can get in the grocery store and the Young Living essential oils is clear to me. I can easily tell which oils come from the plants, and which are manufactured to have the smell. The Young Living essential oils win hands down, every time!