"Magick" is the spelling of the word "magic" that was promoted by Aleister Crowley.
Crowley intended for this spelling to distinguish real magic from the trickery of stage magic.
This Crowleyan spelling was later adopted by various other people, not just followers of Crowley (thelemites).
The spelling "magic" is used by the original wiccan leaders Gerald Gardner, Doreen Valiente, and Alex Sanders.
The spelling "magick" is used by the later wiccan author Raymond Buckland,
and the neo-wiccan authors Silver Ravenwolf and Fiona Horne,
and among non-wiccans it is used by the satanist leader David Myatt,
and by the fat ugly jewish author Donald Michael Kraig.
The spelling "magick" was also used in the 1974 publication of the Earth Religion News version of the long wiccan rede
(probably by the choice of the chief edittor Herman Slater), which was one of the earliest usages of that spelling in Wicca,
and probably influenced subsequent authors to imitate that spelling.
To learn about magic, see the article magic.