Currently the Overworlds surface is populated by the villagers, inconspicuously human-esque life forms who live in buildings made of stone and lumber with agricultural and economic systems. Though I personally feel as though there should be a second kind of civilization on the surface. These being the barbarians, a clan of horned humanoids who act as a distant similarity.
Just like the villagers they build even more primitive structures out of straw and raw lumber. They also show tool crafting capabilities, with most individuals carrying some kind of stone weapon and/or a bow with some arrows. Not to mention they have an agricultural system, choosing to breed animals such as pigs instead of growing fields of crops. And lastly they have chests in which they may keep various belongings, such as loot that may or may not have been stolen.
The biggest difference however is that the barbarians do not enjoy interacting with players. Instead of wanting to trade with you, the barbarian would much rather slay you. Wielding axes that deal a solid 5 points of damage per strike, and carrying bows that deal 3 points of damage per hit. They have 30 points of health, and sometimes wear leather armor. Also worth noting is the fact that they are strong enough to smash right through wooden doors on even the lower difficulties.
Their main purpose in the game is to act as an occupational hazard for early game players. Mostly because they will raid from and kill whatever settlement is unfortunate enough to be close by. It is worth noting however that you can indeed gain some resources from slaying them. Resources such as various food items they may be saving for later, perhaps crafting materials they may have stolen from villagers, or even weapons and other bits of equipment. You also get sufficiently more experience for slaying barbarians than you would from slaying something most other overworld monsters, mostly due to how much of a threat the barbarians are.