A Hunter (Japanese ハンター) is a person that makes a living by hunting monsters and doing requests for civilians.
It is unknown exactly how and when monster hunting started, but it has a been a thing for centuries. One of the very first and most influential hunter was the Hero of Kokoto, a brave wyverian who defeated a Monoblos by himself in his youth. The ancient civilization that once thrived in the Jungle is also known to have experienced the beginning of monster hunting. Eventually, the Hunter's Guild was formed. It is actually an association of multiple hunter's guilds that appeared around the world.
Hunters take on quests posted by the Hunter's Guild, the organization that oversees and regulates their activities. These quests consist of gathering resources or hunting, capturing or repelling monsters. By completing them, hunters protect and help settlements, preserve the balance of the ecosystem and help with ecological research, though some quests are simply requests by civilians. They will also help defend towns and fortresses in case of monster attacks. Hunters are prohibited from hunting outside of quests and authorized expeditions.
To become a hunter, one must register into the Guild, usually via a guild master or guild manager. Although it is not an obligation, most hunters choose to start their career in small villages of the countryside. Every hunter starts with a Hunter Rank, a measure of personal skill, of 1. At first, the only quests they'll have access to involve gathering items and slaying small monsters, often so for multiple months, but eventually they'll be able to hunt their first large monster. As they progress and their Hunter Rank increases, hunters will be able to take on more and more difficult quests involving stronger monsters and more hazardous hunting grounds.
The vast majority of hunters are humans. Wyverian hunters also exist, but are rare. There are a very few recorded cases of felyne hunters.
To ensure hunters don't bite more than they can chew, quests are assigned a quest level based on their difficulty. Hunters may only take on a quest if their Hunter Rank is sufficiently high. Quests levels are separated into three main "tiers": Low-rank, High-rank and Master-rank (also called G-rank). There are also Training quests, even lower than Low-rank, meant for learning the basics.