There are 4 types of magic, Arcane, Divine, Primal, and Eldritch. Arcane is learned magic, based in hidden knowledge. Divine magic is faith magic, drawn from the power of the Gods themselves. Primal magic is nature magic, granted via bargains with nature spirits, elementals, and fey. Eldritch magic is inborn magic, drawn from natural talents and bloodline-granted gifts.
There are 9 schools of magic. These schools are broad descriptors of effect, and serve in contrast to the magic types.
Each magical ability, spell, prayer, invocation, or gift, belongs to one or more of the following schools.
- Abjuration: Magic that disrupts other magic, whether actively or as passive defenses.
- Conjuration: Magic that creates matter.
- Divination: Magic that reveals information.
- Enchantment: Magic which affects the mind.
- Evocation: Magic that creates energy.
- Illusion: Magic that deceives.
- Polymorph: Magic that alters living creatures.
- Teleportation: Magic that crosses dimensional barriers.
- Transmutation: Magic that alters nonliving material.
These 9 schools are juxtaposed against the 8 magical skills. Each skill controls or uses several of the schools above, but no one skill interacts with them all. A well-trained caster usually has at least 2 main skills involved in their magic, while a truly resourceful caster dabbles in other skills outside their own type of magic so they at least have a passing familiarity with the magics of others.
- Arcana (Int): The primary Arcane skill. Arcana is subtle, strange, and full of hidden knowledge. Magics which use Arcana belong to the Conjuration, Enchantment, Illusion, Polymorph, and Transmutation schools.
- Sorcery (Cha): Used by both Arcane and Eldritch magic. Sorcery is raw, magical brute-force, drawing on raw power and fury, but honed by skill to create powerful magical effects. Magics which use Sorcery belong to the Abjuration, Enchantment, Evocation, and Teleportation schools.
- Inborn (Con): The primary Eldritch skill. Inborn is a generally passive skill, based primarily in inherent resistance to magical effects and influence, but is also used for magical effects derived from a physical component, such as draconic breath weapons. Magics which use Inborn belong to the Divination, Evocation, Polymoph, Teleportation, and Transmutation schools.
- Perform (Cha): Used by both Eldritch and Primal magic. Perform is primarily used by Bardic songs, but is sometimes used in shamanic rituals as well. In both cases, the skill of performance triggers the effect, whether through talented musical manipulation of natural forces, or impressing primal spirits with the power and emotion in the performance. Magics which use Perform belong to the Abjuration, Enchantment, Illusion, and Transmutation schools.
- Mystery (Wis): The primary Primal skill. Mystery relates the intricacies of the spirit world and fey creatures, their gifts and knowledge. Magics which use Mystery belong to the Abjuration, Conjuration, Divination, Illusion, and Polymorph schools.
- Mending (Wis): Use by both Primal and Divine magic. Mending is the knowledge of healing, both mundane first-aid and raw healing magic. (A simple Mending check will heal straight HP damage, though it takes between ten minutes and several hours.) Magics which use Mending belong to the Enchantment, Evocation, Polymorph, and Transmutation schools.
- Theurgy (Cha): The primary Divine skill. Theurgy literally means the working of miracles, and even those without levels in the Priest class can manifest their faith in the form of divine power. Magics which use Theurgy belong to the Abjuration, Divination, Illusion, and Teleportation schools.
- Necromancy (Int): Used by both Divine and Arcane magic. Necromancy works with the spirits of the dead and necrotic energy to achieve unholy ends, but it can also be used to disrupt undead, and the skill also covers knowledge of the undead. Magics which use Necromancy belong to the Conjuration, Divination, Evocation, Teleportation, and Transmutation schools.
Mechanically speaking, your total ranks in a given skill are used in place of caster level for many magical effects, or use ranks in the relevant skill as a variable in the formula somewhere. (some cases use an active skill check to determine accuracy or error-rate of an effect, like with dispel checks or teleportation accuracy, the way Spellcraft should have been done.) This means, for instance, a Mage who focuses in elemental combat magic (who primarily has ranks in Sorcery for the Evocation school) can't just pick up a high-end Necromancy spell and be as good with it as anyone else, he would have to have some ranks in the Necromancy skill to make them effective.
This is all kind of an overview (mostly drawn from my numbers charts), but I'm going to try to throw together some examples. If anybody has any questions, or suggestions, or anything, please ask, it helps me flesh things out. (I've got explanations for just about every magic-school/skill association, for instance, I would be happy to expand upon.)