Magic in the world is best divided into three distinct categories - component magic, priesthood and necromancy.
This is the magic practiced by wizards. It involves the mixing of elements (ingredients) together, and then using words as a catalyst to release the energy of those combined elements. It is founded in science and learning, and canbe taught to those of extraordinary ability, as the spells still require the release of personal magical energy from within the practitioner. Additionally, this power can be infused by the wizard into inanimate objects, to create charged wands, staffs, amulets, etc. In the Vitalion Empire, magic is very tightly controlled and regulated by the Wizards Guild, and is used primarily for entertaining the nobility. But among the outlaws, renegade magicians are still valuable members of adventuring crews.
This is the power of the Gods (or other innately magic beings, such as demons) channeled through a worshipper, such as a priest or priestess. The worshipper does not have any power of their own; they are wholy reliant upon the benevolence of their God. As such, this power can be very limited, or extremely powerful, depending on the purity of the connection between God and worshipper, and the will of the deity. Many deities place exacting demands upon their worsshippers, requiring strict obedience in return for access to power.
Necromancy is a rare ability, mastered by only a very few willing to delve deep into dark magics. Practitionaers of this power are able to steal energy from living things, rather than from the combining of elements, and then release that energy in whatever form they wish to conjure. Access to unlimited energy makes necromancers extremely powerful and dangerous, but the practice is not without side-effects - the magic has a corrupting effect on the soul. Marek may be the first, and only, innately born necromancer in history, hence her extraordinary appeal to Szorlok.
One final word on magic - it is Gojun Pye's belief that there is a unified theory of magic to be discovered, and that this is the power that the Gods wield. His theory supposes that utilizing a power akin to necromancy, a wizard could draw energy from any matter by reconfiguring the component elements of that matter into whatever elements may be required for a spell, drawing only from the power within the wizard. It was this theory that drew him initially to the study of necromancy, and also a hope of realizing this dream that draws him to Marek.