There is only one source of magic in the world: the Worldken, a field of raw magical energy that permeates every known plane of existence. It is unevenly distributed and naturally forms ley lines, nodes of magic, dead magic zones, areas of wild magic, and so on. Any intelligent beings can be born with the innate potential to manipulate the Worldken with an effort of will. This is called being Gifted, or having the Gift, and is relatively common in most worlds. Those Gifted who choose to study and develop their ability can become full spellcasters. This might take place through training in an Imperial Wizard’s College, by experimentation as a self-taught hedge witch, or anything in between.
The distinction among classes of spellcasters are generally due to cultural traditions rather than fundamental differences. All draw upon and cast magic; wizards often believe their god to be the ultimate source of their power, while clerics devise scholarly traditions to use their magic more effectively. Each type of spellcaster interacts with their society in their own way, as do the Gifted in general. In some places, such as Zendikar, magic is a divine blessing and looked upon with favor; in others, like much of Earth, it is a mark of devilry to be feared and hated.