What is "High Magick"?

One point of view is that High Magick concerns the ritual manipulation of energies, thoughtforms, and tools to obtain effective, reliable effects. It is also often said to be the commanding of natural spirits. Most commonly, it involves one's own energies, colored and amplified by set, circumstance, and will to achieve magickal action. Overall, this point of view considers high magick to be closely akin to science.
By contrast, low (or hearth) magick concerns communion with divine energy and/or energies and spirits of plants, animals, etc. to obtain what is needed (rather than what is wanted) for oneself or others. Most commonly, this form uses personal energy as a gift to the spirits or to the Divine in return for aid. Essentially, this method is similar to prayer.
Bear in mind that 'high' and 'low' in this case DO NOT mean better or worse; if anything, hearth magick is deeply more satisfying than high magick. Besides, nearly all ritual involves elements of both. In the end, it mostly depends on how 'rational' or 'ecstatic' your beliefs and practices are.

Hope that helps... and doesn't offend...

See Also:
Francis Melville : "The Secrets of High Magick"
Donald Micheal Kraig : "Modern Magick: Eleven Lessons in the High Magickal Arts"
Aleister Crowley/Israel Regardie/SL Macgregor Mathers : "777"

High Magick describes using magick for spiritual growth and development instead of material or earlthly matters like live, money, power or happiness. High Magick is generally concerned with long term goals.

High Magick is Magick concerned with the development of Magick, Magickal systems, teaching others Magick, Etc. This is known in many belief systems as 'The Great Work' - helping others grow in awareness.

It also should be noted that 'High Magick', like all sensible forms of Magick, can be a subjective term.

Great Books on Magick:

Liber Null & Psychonaut - Peter J. Carroll
Liber Kaos - Peter J. Carroll
PsyberMagick - Peter J. Carroll

Also:

Austin Osman Spare - Any & All Works
Some Carlos Castaneda
Bhagavad Gita
David Icke with Shaman Credo Mutwa (Video)
Marilyn Manson - The Long Hard Road out of Hell
Gnostic Gospels

High Magic is powerful type of magic.