What is it?
In the broadest sense, Magick is any act designed to cause intentional change. This term is spelled with a "k" to differentiate it from other practices, such as "stage magic". Magick is not capable of producing "miracles" or violating the physical laws of the universe (i.e. it cannot cause a solar eclipse), although "it is theoretically possible to cause in any object any change of which that object is capable by nature" (Book 4). Aleister Crowley saw magick as the essential method for a person to reach true understanding of the self and to act according to one's True Will.One must find out for oneself, and make sure beyond doubt, "who" one is, "what" one is, "why" one is...Being thus conscious of the proper course to pursue, the next thing is to understand the conditions necessary to following it out. After that, one must eliminate from oneself every element alien or hostile to success, and develop those parts of oneself which are specially needed to control the aforesaid conditions. (Book 4)
"Magick is the Science and Art of causing Change to occur in conformity with Will" "Every intentional (Willed) act is a Magical act." "Magick is the Science of understanding oneself and one's conditions. It is the Art of applying that understanding in action."
For Crowley, the practice of magick, although it equally applies to mundane things - like balancing the checkbook - is essentially to be used for attaining the Knowledge and Conversation of one's Holy Guardian Angel. Since achieving this state with one's 'Silent Self' can be extremely arduous, magick can be used not only to reach that particular goal, but to clear the way for it as well. For example, if one needed a particular dwelling to perform the operation, one could use magick to obtain a suitable home. Crowley stated that magick that did not have one of these goals as its aim was 'Black Magick' and should be avoided.
Why should you study and practice Magick? Because you can't help doing it, and you had better do it well than badly.