Magic is divided into a combination of four things.
Pseudo-science [such as astrology, divination, and alchemy], only in this setting it works.
Rites and rituals that bring good or bad fortune, or manipulate chance. The weakest would be rites that 'bless' or 'enchant' things to perform marginally better then unenchanted versions, or to bring generic luck. Curses and the evil eye would play a roll.
The ability to perceive, speak to, and summon supernatural beings to serve you. Demons, angels, elementals, and djinn being the most common. In all cases, the Magician is not the unconditional master of the beings he commands. Often there is a pact, or an enchantment to control the spirit, or a magical item [like one of the Rings of Solomon] that forces them to obey you so long as you wear it.
And lastly actual supernatural abilities such as shapeshifting, levitation, and ultimately as the very heights of the Art, elementalism.
The average magician is probably an alchemist who can produce salves, glues, greek fire and the like, an astrologer who can divine the broad facts of certain futures with some accuracy, and commands at least one Familiar Spirit, usually serving only as an invisible spy, not an active enforcer.
The High Magician may know the rites to summon, [but frequently dares not!] the stranger things of the world, and likely has a handful of supernatural powers, such as astral projection or the ability to take the form of a horrid creature, or through certain rites to control the weather.