Barbarians are savage and untamed villains. They are often cruel, dangerous and bloodthirsty with a desire to rule by force alone. Although stereotypically strong, they don't have to be strong to qualify. Basically, a barbarian fills the role of a semi-feral being intelligent enough that they still retain some humanity but are still ruled mainly by base instincts such as war and aggression.
A barbarian can also be a rude, violent and sanguinary individual. In some cultures, they are known as "ogres" (a reference to the mythical monster of the same name).
Another common trait of a barbarian is strong anti-intellectual sentiment, though this is not a universal trait as some barbarians can be intelligent, even cunning.
In other words, Barbarians do not listen to reason whenever heroes say they should.