Essential oils are aromatic compounds made from steam-distillation or cold-pressing of a plant (bark, seeds, flowers, or leaves from a plant). Each drop has active constituents that help support life in our bodies. They themselves are not alive but they help enhance the life that is already there.
The volatile compounds in oils are often times 80 times more concentrated and powerful than an herb. Although they are currently trendy (note not all essential oils are created equally), they have been used for thousands of years.