Hunters (헌터) are humans who possess magical powers and use them to fight and kill magic beasts. They first appeared around the same time humanity was exposed to the existence of magic. As hunters turned out to be the only ones who could kill magic beasts and close gates, a global billion-dollar industry eventually developed around them as a result.
Hunters are ranked by an international S-E system to show how strong they are, with E being the weakest and S being the strongest, and possess a wide range of abilities. There are also two general categories of hunters, combat and non-combat, and six different specific types of hunters: Fighters, Mages, Assassins, Tankers, Rangers, and Healers.
It should be noted, however, that hunters and humans with powers are not always the same thing, as humans only become hunters when they actually HUNT. As such, there are plenty of humans with powers who chose not to become hunters and pursue normal civilian livelihoods instead. Good examples seen in the story include Yoo Soohyun, who became a famous model, and Han Song-Yi, who decided to stick with finishing her education.