There are two terms tossed around a lot in the occult, esoteric, and pagan communities, sometimes with derision, sometimes in an attempt to explain differences. Sometimes they are used descriptively, to define what is observed, sometimes prescriptively, to try to force a designation and discourage the practice of one or the other. As with all labels, these terms can be restrictive or insulting if used that way, or useful in understanding used a different way. While I discourage the use of any label to restrict or insult another, I find labels useful if they are used to better understand the subject without restricting it. You get a bit of the wave or particle issue in quantum physics. Can you apply the label without changing that which you are applying it to? Can you observe that which you are labeling without changing it?
For a label to be useful, we must first understand the label itself. What is meant by it? The labels in question here are High Magic and Low Magic. What do we mean by them? They can also be called Superior Magic and Inferior Magic, but these terms have even more loaded meaning due to current understanding of the terms. Superior means “above”, inferior “below”. These have taking a meaning of above or below in value or in social power or in strength, but this was not the original meaning. I’d point out a maximum used often at both ends of the spectrum. “As above, so below, as below, so above.” High Magic and Low Magic are different in form, and often in goal, but not in essence, not in principle.
The meanings have changed since the original usage. High/superior does not necessarily mean better or more advanced, nor low/inferior mean lesser or more savage. Those meanings can be blamed on the Victorians. Consider a similar pair of terms (and actually having the same meaning as here), High Church and Low Church.
High Church (n)
(Christianity / Anglicanism) the party or movement within the Church of England stressing continuity with Catholic Christendom, the authority of bishops, and the importance of sacraments, rituals, and ceremonies
Low Church (n)
A group in the Anglican Church that minimizes the episcopacy, priesthood, and sacraments and favors evangelical doctrines, polity, and usages.
Basically, High Church is religion focused around set ritual and ceremony, with a priesthood and hierarchy. Low Church is religion minimizing those aspects, more ab lib and unstructured, more focus on the laity and non-ritualized worship.
Same goes for High Magic and Low Magic. High Magic is ceremonial magic, with set ritual and form, passed down through structured teaching. Low Magic is the more non-structured folk magic.
Another similar set of terms is High Fantasy and Low Fantasy. High Fantasy is a fantasy sub-genre where the events or characters can’t exist in what is considered the laws of physics in our world. Elements like magic, dragons, elves, etc. Lord of the Rings is High Fantasy, as are the Mists of Avalon. Low Fantasy is fantasy that fits within the “real world”, usually historical with minimized supernatural elements. Both Mary Stewart’s Merlin books and Stephen Lawhead’s King Arthur series are closer to the Low Fantasy end of the spectrum. I’m not saying High Magic is impossible fantasy. High Fantasy is more airy, Low Fantasy more earthy. High Fantasy typically uses the fantastic elements to facilitate the story, like High Magic and High Church using ritual to facilitate the worship or magic (and there’s a fine line there), often focusing on inner transformation and more abstract gains and goals. Low Fantasy typically relies less on the fantastic and more on the day-to-day of living, like Low Magic and Low Church rely more of folk belief and practice, more on magic or religion for addressing mundane needs.
High Magic is an expression of ritual form and hierarchy, not a value judgement. Low Magic is an expression of more every-day and less ritualistic practice, not a value judgement. While many might use them as judgement, they are useful designators of two ends of a spectrum of form within magic and occult and esoteric practice.
*Definitions from http://www.thefreedictionary.com/