Adventurers for hire are those who work on commission to do quests and missions for those in need.
Most wannabe adventurers stay simply at that, and are not able to make a proper living doing quests for people.
There is an organization that adventurers can join to be listed for hire and by extension display their rank and their specialties. There is a tier system in place to rank the level of skill any particular adventurer or adventuring group falls into. This is to make sure that beginning adventurer groups don't get sent to their deaths.
Lower ranked adventurers usually only get sent out on simple fetching quests that most ordinary people aren't able to carry out, while higher level adventurers might be hired to slay a beast or take down an evil organisation.
Treasure hunters and thrill-seekers aren't employed by anyone, and are usually motivated by greed or pride- but not always.
These people go on missions to uncover ancient artifacts and treasures, as well as slay any beasts that they may deem worthy of slaying. This is usually to seek fame or praise of some kind.
This group tends to be less skilled than the adventurers for hire.
Heros tend to be self employeed adventurers who do good simply to better society. They are not always paid for their acts of service.
Heros are often praised and reveered in their community.
This group tends to be more skilled than both of the previous two.