Magic is best described as the "supernatural all." It lives and thrives in all things. Some beings possess greater levels of it, and are able to manifest it with the snap of their fingers or rhyming of words, or project their imagination to reality. However, some beings take a long time to master magic, if they have it, while some are gifted naturally, such as the Cromwell family.
First of all, Magic never discrimates between good and evil, it is neutral, and therefore the use of magic, and the blame or good deeds it deserves, is reserved solely for the user. It is at the user's discretion that magic is accessed. Marnie is a prominent Cromwell witch.
Warlocks and Witches are the most common types of magical beings, however their are such beings such as practitioners (although they are really mortals) that "play around" and "mimic" the craft.
Witches are born with their powers and can learn witchcraft better when they start their training.