So far it seems that most humans in Kaelaross are survivors of the four collapsed empires. Large though the Empires were, they did not cover all the land of Kaelaross. Once the Empires had established themselves, most of the barbarians, especially those neighbouring the Empires, were pushed aside, conquered or assimilated. By the time of the Wars between the Empires, the remaining barbarians had been marginalized to areas neither profitable nor strategically important. With the collapse of the Empires, the barbarian tribes are more free to expand and reclaim their old lands, and to plunder the ruins and survivors of the Empires. After centuries of conquest, persecution, exploitation and punitive raids there is little love lost between the barbarians and the empires.
The barbarians have never embraced Law much - they are usually neutral or sometimes chaotic. Their attitude towards the forces of Chaos that have emerged from the Chaos Portals varies. Some have pledged outright allegience to one of the Chaos Gods (Slargor is popular among barbarians). Others view Chaos as an ally of convenience, others are rivals to Chaos. Some barbarians (particularly the neutral, less bloodthirsty tribes) consider the forces of Chaos as an enemy to be destroyed, at least as dangerous and destructive as the Human empires.
In terms of B/X D&D and Labyrinth Lord, human barbarians are generally fighters, though their scouts may be thieves and their shamans are treated as clerics. Magic users are a lot rarer than in the remains of the four empires, and some barbarian cultures do not permit magic users at all. Barbarians are notoriously wary of wizards, and some are violently hostile to them.
As an optional rule, DMs may allow barbarians who shun all armour except shields to double their dexterity bonus to AC (this has also been used for swashbucklers who do not wear armour either). As most barbarians are fighters, they can wear armour, even plate mail, if they wish, but then they lose the additional dex bonus. Some folks might think that they should be allowed to keep the double dexterity bonus while wearing leather armour, but I disagree. The Celts and the Germanic tribes who fought the Romans shunned all armour, and Native Americans, South African Zulus, jungle tribesmen from the Congo and Amazon and most Central Asian horsemen (Huns & Mongols) did not wear armour. Most warriors who wore leather armour were not barbarians as I understand it.
Although not the stereotypical barbarian, demihuman tribes are sometimes included, particularly when they have shunned Imperial lifestyle and authority. The elves and centaurs of the Shorgan Forest fit this description.
Humanoids such as orcs, goblins, gnolls and kobolds are not normally classed as barbarians but as monsters, as they are considered inherently chaotic and unable to become civilized. Whether this is absolutely true is a matter of debate, but so far all attempts to civilize such races have failed badly - you can teach a kobold how to hold a knife and fork correctly but the little bastard will still try to stab you with them. Barbarians from a Kaelaross point of view are uncivilized members of a civilized race
The known realms of human barbarians include:
The Siluri of the Talloak Forest in between the Godsblood Straits and Iryanland - they are now a shadow of their former might, when they used to attack and raid the Dwarves of the Ten Peaks.
The Cheruscu of the Twilight Forest in the Walrus Freehold
The Garavu of the Twisted Hills in the Walrus Freehold