Magic comes in the form of magic items, spells, and rituals.
Although not magic, there are also spell-like effects which are similar at the end of the day.
Offensive magic may require a MAG check to see if they hit, or alternatively (not both!) the target may roll AGI to see if they dodge.
Types of Magic
MAGIC ITEMS can be used by anyone, though most magic items are picky about who can wield them. Magic items have beneficial effects and/or grant special abilities to their wielder. They do not require a MAG score to use.
SPELLS can only be used by those that have studied magic, represented by a MAG score. Those without a positive MAG score cannot cast a spell.
You create your own repertoire of spells, which you should keep track of. Spells can be learned from a teacher, studied from books, or developed on your own. As a spellcaster gains levels, they can buy a new spell instead of increasing their stats.
Spells typically have simple effects and appear instantly, such as a fireball, a lightning bolt, a small heal, a message delivered. In general, they may have any effect you choose, though stronger and more elaborate effects are more likely to be rituals (see below) than spells.
SPELL-LIKE EFFECTS are not spells. They are special skills that seem superhuman, and they do not require MAG. Their effects are seemingly unnatural, often the result of extensive training, but they are not actually magical. For example, a ninja may have smoke bomb or hide in plain sight as spell-like effects. An ancient bloodline that practices meditation may know secret techniques for increasing STA or POW as a buff.
For the purposes of character creation, leveling up, and acquiring new spell-like effects, they follow the same rules as spells.
RITUALS can be used by anyone, but they require a MAG check, so characters with high MAG are able to use them more reliably.
A ritual requires material components—a magic circle, holy water, a voodoo doll, whatever might make sense—and a lengthy incantation. A ritual is not meant for combat and should not be cast on the fly. Upon conclusion of the ritual, a MAG check determines whether the ritual was successful. If the MAG check greatly exceeds the check, it may have extra and unforeseen effects.
Rituals can be for any purpose. Demon summoning rituals are the most (in)famous, followed by weather changing rituals, which are highly prized by many communities.
There is no particular method for acquiring rituals. They may be bought like spells for a +25 bonus to MAG checks to perform the ritual, but instructions for rituals can be found in libraries, long lost tomes, from mentors, or researched and developed from scratch. They are free, as long as you can explain (through background or in roleplay) how the character learned the ritual.