"Like a mule, neither horse nor donkey, yet possessing the qualities of both, a Witcher stands somewhere between a mage and an ordinary person.
Witchers can cast simple spells they call "Signs," using basic telekinesis (Aard), pyrokinesis (Igni), hypnosis (Axii), shields (Quen), magical traps (Yrden), and so on.
However, Witchers can't use more complex spells, and they don't dare put much faith in true magic. Witchers generally have an irrational, superstitious fear of teleportation (I'd like to remind you that fatal teleportation accidents only happen once in a hundred times!).
The reason for this aversion is obvious: Witchers aren't born with magical abilities; they acquire them during the Trial of the Grasses. So their spells are actually unnatural—and, in fact, everything about Witchers is unnatural."
Appendix: The following content was compiled and added by Vesemir.
Igni (Flame): An active Sign that can spew fire to incinerate enemies or be used to light firewood, bombs, or campfires. It's also an effective way to fight heavily armored knights.
Quen (Earth): A passive Sign that forms a magical shield around the Witcher, absorbing a small amount of physical and magical damage.
Yrden (Aether): An active Sign that creates a magical trap on the ground. Opponents caught inside will be slowed. This Sign can be enhanced to produce additional effects.
Aard (Air): An active Sign that releases a telekinetic shockwave to repel, stun, knock down, or even disarm opponents. Aard is especially effective against large groups of enemies.
Axii (Water): An active Sign that can influence another person's mind, charming and controlling them or calming a frightened horse.