According to Belgarath in Enchanters' End Game, chapter six, he describes magic this way, "The magician tries to control demons with spells, formulas, incantations, symbols, mystic diagrams - that sort of thing. As long as he doesn't make any mistakes, the demon is his absolute slave and has to do what he tells it to do. The demon doesn't want to be a slave, so it keeps looking for a way to break the spell." And if it does, "It generally devours the magician on the spot. That happens rather frequently. If you lose your concentration or summon a demon too strong for you, you're in trouble."
In chapter seven, Belgarath duels with a Morind magician known only as "White-braids," and magic is seen in more detail. First a five-pointed star enclosed with a circle is drawn, into which the magician steps. This pentacle protects the magician from the demon they summon. Belgarath and White-braids then pronounce an endless chant of incantations, which summon and direct the demons. After the duel, Belgarath explains that "It's the shape that's the key to the whole thing . . . You form an illusion drawn out of your imagination and force the spirit into it. As long as you can keep it locked up in that illusion, it has to do what you tell it to. If the illusion falters for any reason, the spirit breaks free and resumes its real form. After that, you have no control over it whatsoever."
In Belgarath the Sorcerer, when Belgarath goes to learn magic from the Morindim, he lists the various types of demons in ascending order of difficulty/strength: imps, fiends, afreets, and finally full demons. This isn’t an exact science, as it’s the only reference to such “levels” of demons. He also lists the kinds of “tricks” that full-sized demons can perform: “turning water into blood, setting fire to a rock, withering an acre or so of grass”  but the exact limits of a demon’s powers are unclear.
- Belgarath the Sorcerer, Chapter Nine