Aromatherapy is defined as the art of using specific scents to bring about a desired effect on the mind, body, or soul. We have instant responses to some scents because of association, but others, more unfamiliar to us, can cause change in our system, because of the properties, or frequencies of the scents. I won't go into great detail on the frequency of scents or oils, or anything else, simply because I don't quite understand how they work, but a few words must be said to make this subject make sense.
Our body, and it's systems and organs, has a certain "normal" frequency. Whenever we are affected, be it negatively, or positively, by something, it changes that frequency. Toxins, illness, emotional extremes, and injuries all affect these frequencies. Essential oils and fragrances also have certain frequencies. To be used effectively, the oils or fragrances used, should be similar to the normal frequency of the system it is being used on. In a very compact nutshell, like attracts like.
Now, I know that I don't have the time, or truly the desire, to learn the frequency of every oil and scent that I use, so I learn other things, like which groups affect the immune system, the circulatory system, the nervous system. Which groups have neurological effects, which have psychological effects. That is how we will learn to classify the fragrances we will use during these lessons.
A word about fragrances and essential oils. In my experience, for true aromatherapy, fragrance oils (manmade oils) are fine, because the actual scent is what will cause the effect we are seeking. If you are planning to use the oils topically, or internally(though the FDA has not approved this) please use only pure essential oils. The obvious exclusions to these rules are, of course, in soaps and body or massage lotions. These products often contain artificial or blended scents. While they will have no chemical beneficial effect, the scent will still be thouroughly delicious, and will serve wonderfully.