High magick is ceremonial or ritual magick, used more by the practitioners of The Order of the Golden Dawn, Kabbalists, and followers of Aliester Crowley. High magick also follows a set of guidelines and certain formalities. Ceremonialists call upon demons and other entities to do his/her bidding; this ritual takes time and needs to be done correctly. These guidelines follow a very precise set of parameters and if deviated from usually cause problems for the practitioner.
There are also other references to the differences; one of the more common ones is that High Magick came from those that lived in the cities, castles on the hill. It is said that those who practiced High magick had more time on their hands, therefore their rituals are more expansive. Low magick is practical everyday, or folk magick, generally used by wiccans and witches, and other nature-based religions. Low magick has less formality and encompasses more of the "common sense" magick. Low magick often involves the use of herbs, aromas, stones, flowers, animal parts (sympathetic magick) and other natural materials. These animal parts are NOT the result of sacrifice, but rather the practitioner finds that feathers, furs, and discarded bones. Low magick came from the fields, and the lowlands. The fact is that most of those who lived in the lowlands worked from dusk to dawn, had little if any time or need for elaborate ceremonies; therefore the practice of low magick was straightforward and to the point with little fanfare.
Books You Might Enjoy:
Eliphas Levi - The Ritual Of Transcendental Magic
Phillip Williams - Night Magic
Thomas Moore - Candle Magick For Love