Oils are volatile aromatic compounds. This means that they waft into the air (the reason you can smell a lavender bush as you walk past it). They're extracts from plants found inside roots, seeds, flowers, bark.
Now, my favourite part - they help keep the plant safe, warding away harmful pathogens and assisting with pollination - so you literally have nature's protection right there in those pretty little bottles. They’re concentrated, highly potent chemical compounds (wah! Yes, nature has natural chemicals, they are still chemical compounds) which act as messengers within your body, giving instructions to your cells. Interestingly, a plant's cellular structure is similar to a human cell which is how we can benefit so well from these natural oils.
Each plant has its own unique chemical makeup, the benefit of which is determined by the plant's chemistry. This is why we ‘need’ so many different oils and why different people react more or less to one oil or the other. You can tailor make them according to what suits you best, without side effects.
Isn’t nature smart?!