Barbarians are warriors. They aren’t formally trained to the degree of Fighters, Paladins, or Monks, but often number among martial ranks and have at least some working knowledge of combatant routine.
Barbarians are able to bear a great deal of pain and injury without slowing down, and they can inflict more damage on average with each attack than other warriors. Some barbarians achieve their greatest results by going into a berserker/rage state, which carries the risk of growing indiscriminate about who gets targeted.
While some cultures consider barbarians to be brutish liabilities, many others hold them in high esteem as specialists.