So how does magic work? How does the system work, who can use it and what can they do with it?
Assuming you mean Sorcery:
Each spell costs a number of Sorcerous Motes to cast, as well as at least one point of Willpower to start casting.
You gain Sorcerous Motes by taking the Shape Sorcery action, which is a combat action that can't be placed in a Flurry but otherwise does not drop you out of combat or make you defenseless. You then roll (Intelligence + Occult) and gain that many Sorcerous Motes. If that reaches the cost of the spell, it is cast. Otherwise, take more Shape Sorcery actions - if you don't, you loose three sorcerous motes per round.
When you succesfully cast a spell, you gain a point of willpower so most spells do not in fact cost you any willpower unless you get interrupted.
Shaping Rituals can provide free Sorcerous Motes, you learn them by initiating into a Circle of Sorcery (so you always know at least one and up to three, maybe there will be ways to learn additional ones later on?).
If you made a Bargain with Mara to learn sorcery, you might be able to draw on the unrequited love of others toward you to gain extra sorcerous motes. Or you could have learned how to harness the energy of burnt offerings, or how to brew elixirs that catalyze your ability to shape sorcery.
Those Shaping Rituals also open access to merits, for example if you made a Pact with an Ifrit Lord you could be able to call forth flames at will - usable as a weapon and for utility.
Spells that take long enough to cast that it's not feasible in combat don't bother with the Sorcerous Motes system, they just state how long it takes to cast them.
Then there are Control Spells - your signature spell(s), those that you learned first and are most familiar with. That usually provides a bonus of some sort - various Shaping Rituals are more efficient if used for your Control Spell, and various spells gain additional functions if they are your control spell. Having Infallible Messenger as a control spell for example allows you to perceive through the senses of the summoned Cherub, instead of it just being a magical telegram.
The real nifty part though is that your control spell usually changes you. If you have Death of Obsidian Butterflies as your control spell, shadowy butterflies might spill from your shadow when you are agitated and your fingernails turn into obsidian when you are angry. Invulnerable Skin of Bronze would give your skin a Bronze Taint. If Magma Kraken is a control spell, nearby flames will curl around you and small firey tentacles might appear around you when your emotions run hot. If Impenetrable Veil of Night is your control spell, you are always partially cloaked in shadow and speak with a distorted voice.
Which is really nice because it makes Sorcery
weird, which it was always supposed to be fluff-wise. Though if you don't want permanent changes there are plenty of spells that don't cause them.
Terrestrial Spells in the core book:
Cirrus Skiff summons a cloud you can ride and control.
Corrupted Words prohibits the target from communicating one thing by any means, if they try they throw up violently.
Death of Obsidian Butterflies summons a swarm of obsidian butterflies. A good combat spell against large groups, not so good against single targets.
Demon of the First Circle summons and binds a First Circle demon for a year and a day.
Flight of the Brilliant Raptor launches a bird made out of fire at an enemy, exploding into flames and igniting everything around it.
Infallible Messenger creates and sends a small intangible cherub that can only be perceived by the target, to whom it delivers a verbal message.
Invulnerable Skin of Bronze does what it says on the tin, greatly enhancing your durability.
Mists of Eventide saps the initiative of those affected by it, plunging them into a day-long sleep if its effective enough.
Silent Words of Dreams and Nightmares allows you to influence the dreams of a target to whom you have a symbolic link (lock of hair etc.)
Stormwind Rider creates a dust devil that can carry you and several other characters at high speeds.
Summon Elemental doesn't actually summon an elemental, but creates a relatively weak one on the spot. Which then serves you for a year and a day.
Wood Dragons Claw transforms one of your hands into a wooden claw, which can be used for some nasty attacks.
Then there's Celestial and Solar Circle spells, which i may cover later.
In addition to all that, Sorcery can also be used for long-term projects which can do anything from creating a new spring of fresh water over permanently altering the weather for a whole town-sized region to causing a city to fly or outright altering the metaphysics of the entire world.
