Thinking about the three basic types of magic (there are more, but these are the most common):
Spirit magic is a song, blessing, chant, trick, talent, etc., that gives you an advantage. It is clearly not the power of a god. Varanis used his trick that made him hard to see, like a shimmering blur; Vasana rubbed the side of her sword with her hand, and the glean of Bladesharp became visible; Yanioth sang to the spirits to heal Vishi Dun’s wounds; that sort of thing. It is the most common magic in Glorantha.
Rune magic in contrast let’s you briefly wield some of the power of the god. When you character uses True Blade, it is fair to say that Humakt was present at the combat. Use enough Rune magic, it is fair to say that the god appeared and fought.
Sorcery in contrast is knowledge and will. The caster imposes themselves upon the mundane world. A sorcery might study the metaphysical workings of the cosmos for decades in order to glean how to combine Fire and Water and summon the greatest possible manifestation of that combination.
Spirit magic and Rune magic appear in most cultures, which makes sense in a setting with omnipresent spirits, and gods that made, broke, and revived the world. Sorcery is less common, and is widely prevalent only in the West (where it is the province of the Zzaburi caste), Fonrit, and Kralorela.
