Magic is the 'technology' of Haerth -- as Information and Manufacturing are in ours. Much simplified, there are two or three main sorts of practitioners:
Magicians -- Learn how to use existing non-physical 'machines' -- 'Spells', that take your magical energy and perform tasks for you. Simple to learn, not too flexible, really kind of dumb 'black-boxes' unless you are a good scholar.
Sorcerers -- Use complex rituals to summon powerful magical critters from other parts of the Cosmos to work for you. These tend to be either Elementals or Shadowspawn, both being smarter (and more willful/self-directed) than your average Spell.
Pattern-Magic (Chantry) -- This involves repetition of patterns (usually as part of the creation of some physical artifact) so that the artifact takes on subtle magical powers. To folk in the Kingdom, this actually is not so much of a magical discipline, but more of "that's just the right way to make a ______". For example, most buildings have some sort of pattern painted into the walls as protection against demons, spirits, and misfortunes general. Most practitioners don't think of themselves as magicians -- more as craftspeople.
Q: What kinds of magic exist in the Kingdom?
A: That is a complicated question. In broad outline, they fall into the three categories outlined in the Primer: Spells (Mage-work, “Spell Slinging”), Sorcery (Summoning), and Chantry (Pattern Magic). I tend to lump a lot of ritual magic into the Chantry or more correctly, “Pattern Magic”. Ritual Magic also shows up in the other categories (usually as an aid in performing the main work).
Q: Who can learn magic?
A: Anyone who has (a) Time; (b) Money & (c) Access to the knowledge. If we are talking about Mage spells, that means (usually) being part of a Cult, or in a Guild (typically the Magic Users Guild). The Magic Users' Guild even offers Associate Memberships to those who want access to (most) 1st Rank spells, but do not want to be bound by the Guild Rules, or to join a Cult. Sorcery takes a great deal of time to learn, is chancy to perform (it's a skill you practice), but gives you some pretty powerful effects. Even better, the things you summon can act independent of you (spells usually need to be cast directly by you). Sorcery skills are usually learned (initially) under the authority of the Magic Users Guild, either directly in the Guild Hall, or taught by an instructor (in either case it costs money). Traditionally Sorcery is taught through an apprenticeship program. Experieced Sorcerors are able to conduct research, experiment, and learn on their own. Shamanisim is rarely learned (as such) outside of Barbarian tribes.
Q: So if I'm in the Martial Caste, I can actually do magic?
A: Heck yes! If you don’t know at least a few basic mage spells you won't be an effective warrior. You may still concentrate mostly on physical fighting. You may end up as a specialized combat mage. You may instead become a combat sorcerer (or combat-support sorcerer). Or you may be a bit of all three. Contrarily, a Combat Mage (especially one from the Martial Caste) better know the basics of physical combat. The downside for Martial Caste types is that any un-enchanted metal armor you wear (and, to a lesser extent, carried metal too) reduces your chance to successfully cast a spell.
Q: How does Power (POW) work?
A: Assuming you’re not talking in a metaphysical sense (that’d take some time to explain) your Power represents how "networked" you are with the metaphysical parts of the world, influencing your capacity to work magic, as well as your general luck in life. In game terms, performing magic of most kinds will require spending power of some sort: for most cases you will be spending Magic Points (“MP”s)- same number as your POW stat (but see "Power Storage Crystals" and "Bound Spirits"). MPs (once used) will normally return at a rate so that all of your MPs expended will be back in 24 hours. Some magic requires the sacrifice of Power points- permanent Power. These points do not return, but they may be regained in play.
Q: What are these Power Storage Crystals of which you speak?
A: These are shiny black crystals w/ a fine tracery of silver lines w/in them. They measure about 1-2 cm across. There are various stories about where PSC's come from: Some say they are the tears of Kura, goddess of the night, shed during the War In Heaven. Others say that when Ardenalaha was slaying demons, he knocked holes in the night sky (leaving behind stars) with his spear when he missed a demon. Either there were a lot of demons, or Ardenalah missed a lot.
Most are simply a battery you can use to store Magic Points (i.e) drawn upon for spell casting, etc.. You simply dump MP's into it when you have time & spare MP's (say the day before a battle) & then they stay in the PSC until you need to draw upon them. Most PSCs do not generate MPs of their own. However, see “Bound Spirit”, below. Some PSCs are special: “attuneable” crystals have special powers of their own, and cursed PSCs have their own, uh “powers”. PSC’s are rare(ish) and very valuable- even a 1 point PSC is worth thousands of silver pieces.
Q: What’s a Bound Spirit?
A: Since magic users have a limited reservoir of MPs to work with, they can obtain spirits and bind them. Usually that’s done with a Common Magic spell. The spirit may be obtained from a cult or a guild (safest) or even by binding a "free-range" spirit. Spirits may be bound into special cult objects, some special natural objects, PSC’s, or living creatures - a "familiar" - an animal which has had its superior intellect awakened, or its natural spirit replaced with another spirit. The familiar has (in addition to it’s normal, very limited, spiritual senses), all the mundane powers (senses, locomotion, attacks, etc.) of the critter its put into, along with the (usually) greater INT and POW of the spirit. Of course, the Familiar’s mortal body may get killed; you lose the spirit (including use of it’s POW & INT- but not the actual knowledge of the spells), as well as the body.
Bound Spirits do not actually cast spells themselves, they just provide a naturally replenishing source for Mana (POW), as well as storage INT for spell-knowledge.
Q: So, what if I know more than my INT in Common Magic Spell Points?
A: You certainly know the additional Spells; you just cannot keep them in your head well enough to cast them right away. If you don’t have enough points of INT between your native INT, any Spirits' INT, and (rarely) Attuned Magic Crystals with spell storage powers - those additional spells are not immediately accessible. Getting to those Spells requires that you meditate to "swap" un-stored spells at a rate of 1 point per hour. Then, you have a new list of spells immediately available.
Q: Sounds OK - but is there more powerful stuff?
A: Sure. What I’ve described before is mostly Common Magic Spells. There are also Rune Spells. Those are more powerful, faster to cast, often have a longer duration, and are “smarter” taking less direction from the caster. Points of Rune Spells don’t even count vs. your INT. People who become more advanced members of religious cults (Initiates, Lords & Priests), are able to get access to Rune Spells from their god or goddess. Now, the bad news: Rune Spells require that you spend POW (not Mana) to acquire them. Depending upon how you got the Rune Spell, and your rank/ experience, the Rune Spell may be single use: you would need to spend another point of POW to get the same spell again (typically Initiates, Rune Lords). Advanced members of the Magic Users Guild may also obtain access to Rune Spells, first as single use, sometimes later as reusable spells (though that is something of an adventure in itself). Worse, some Rune Spells are never re-usable for anyone, and some are only re-usable for priests of a particular cult.
Sorcerers learn how to summon and bind powerful creatures (mostly from other planes of existence) that may do the bidding of the Summoner. This Summoning us based upon skill in various types, and generally takes years of study, hard practice, and dangerous experimentation to learn. The various types of Summoning Skills are detailed in the Sorceror FAQ.
Pattern Magic / Chantry allows someone to persuade various free range “spirits” to do useful things for the magic user. Like Sorcery, Pattern Magic tends to be skill based. The Mage’s Guild does offer training in some aspects of Pattern Magic in the form of Ritual Magic: it has the advantage over Spell Slinging/ Common Magic in that you have some ability to get at the actual magical effects and tinker with them, rather than simply casting a Common Magic spell as a "black box". The disadvantage of Ritual magic is that it takes time to learn, and it is easier to make a mistake. Some of these rituals are simple folkways for good luck; some are fancy bits of calligraphy inscribed in sand, or stone, or, even the air itself. Some are vast constructions created over long spans of time. Some are simply "built into" various items of art or everyday utility (from tea cups up to city walls). In truth, a lot of the important things that get crafted in the world have pattern magic built in. The word “chantry” itself derives from the elaborate chants that sword-smiths use in counterpoint to their metal beating hammer blows to encourage useful Virtues to voluntarily take up residence inside particularly well-made blades.
Oh, and don’t get me started on the Shaman of the easterling Barbarian tribes who bargain with various free-range Spirits, both to learn spells without a Guild support structure, or to simply have a powerful spirit willing to do the Shaman’s bidding.
Q: How d I get around the bad effects of metal armor on my spellcasting?
Use non-metallic armor. Cow-hide and more exotic leathers (say, wyvern) have no effect. Also, the "stuff" that Sorcerors summon ("Shadowspawn" isn't metallic). Or, use enchanted metal armor. This allows the metal to adjust itself magically to you so that it does not affect your spell casting ability. Metal armor may be enchanted either via Chantry Rituals, or via a godly miracle (usually for a cult's Rune Lords and Rune Priests).