Magic

Felix's explanation
Think as if the world is made of two different places at the same time. Like the surface of a lake. If there is a reflection, there has to be something that projects it. But everything that shows up on the lake has to have a reflection. Well, our plane, the Material Plane, is where the material body is. You can think of it as the object that faces the lake. Meanwhile, the other plane is called the Ethereal Plane, and it's where the Aura is. The Aura is the reflection of material things. The Ethereal Plane is not real in the strict sense of the word. It's more like... a reflection of the Material Plane. Everything that happens on the Material Plane influences what happens on the Etheral Plane.

Now, the thing that we call magic is basically inverting this process. Magic is changing the reflected image, so that reality is forced to follow through. The Ethereal Plane is not just influenced by physical events. Emotions, ideas, willpower, and things like those can also leave an imprint on the Ethericum — which is another name for the Ethereal Plane— and can influence it just as much as physical events do, if not even more. Basically, if you will something to happen hard enough, it will happen on the Ethereal Plane, and then by consequence it will happen on the Material Plane too.

The are barriers to this such as the Veil. It's basically a wall between the Material and Ethereal plane. In some places, the Veil is so thick that magic is all but impossible. In others, it's so thin that magic comes almost naturally. Focus instruments not only allow one to pierce the Veil more easily, but they also naturally modify the flow of the Etherium. All you have to do is channel your aura through a specific focus and you cast a spell.

Usually every species has a different talent for magic. Pretty much all humans can potentially learn some magic, but beastmen are almost completely magically inept, and Djinni are restricted to their innate magic.

Innate Magic
Some innate types of magic (like the deer's antlers and the goblin ability to evolve) seem to act in a peculiar way that can't be replicated (except by using their body parts as catalysts). Most dominant races in the world do have some kind of innate magic that is exclusive to them, and only humans and few others lack it... though they have developed sorcery to make up for it.

Aura Users
Aura users use their Aura to directly enhance their bodies. Compared to sorcery, Aura Arts are much more difficult to perform but also much better suited for combat.

Aura Orders
Groups of Aura Users who work together doing mercenary work or acting as part of a nations defences.

Most Aura Orders do mercenary work anyway, even the ones who are associated with the Church of the Holy Light. There are exceptions of Orders that do not do mercenary work, but those are usually funded and controlled by a state (like the Medelanian Royal Knights)

Known Orders

 * Order of the Rose
 * The Royal Knights of Medelan

Rankings/Grades

 * Granted, she technically wasn't an Aura Master yet but that was only because she had postponed her trial in order to take part in this godsdamned expedition, and she was more than skilled enough to hold the Crimson Sheath by now. Prologue Rewrite
 * Three hundred men, the elites of Medelan's army, all at least Yellow Sheath Aura Users, hand-picked and trained by Yasen himself. Ch. 104

Sorcerers
Sorcerers channel their aura through focus instruments. Sorcerers couldn't really use magic without their tools, so a lot of them, Zeno included, kept various focus instruments hidden in their clothes in case of emergency. Ch. 39

Focus Instruments
Focus instruments not only allow to pierce the Veil more easily, but they also naturally modify the flow of the Etherium. All you have to do is channel your aura through a specific focus and you cast a spell!

Known Focus Instruments

 * Flare focus
 * a small, shiny stone, wrapped by a thin copper wire.
 * Lightning focus
 * a long straight white horn coiled in copper wire.
 * Sleep Focus
 * a voodoo-like fetish that looked like a mummified hand with six unnaturally long fingers with multiple knuckles tied together by a black string

Shamans
Shamanism is a very crude and primitive form of magic, but it can still be quite powerful. The main weakness of shamans is that they can only use their powers in a limited area. The further they get away from their home, the weaker they get. Ch. 46

Shamans use familiars but in their case their familiars are local spirits they have bonded with (Ch. 99).

Divine Magic
Magic related to churches, gods and religion used by clerics (Ch. 105).

Mages Exhaustion
Caused by trying to cast a spell too powerful for their aura. In less serious cases the treatment is just to make the patient drink water with some nutrient powder mixed in it, in order to keep them hydrated and replenishing their energies.