Barbarians are a touchy subject. When the term is used correctly, they are semi-nomadic, lack a visible set of laws, and raid and pillage for their livelihood. When it isn't used correctly, they are cultures that have less developed technology and different standards of beauty, law, social expectation, and so on. We will be talking of barbarians of the first kind.
Barbarians have two reasons for raiding and pillaging. The first one is purely empirical: the need for material objects. To this end, their ruthless tactics might be explained by "gaining reputation," so as to frighten the villagers when they attack and make the battle that much easier. Their other reason is not so well supported: the desire to create an "Empire." Everyone hates insults about them, for example, a male will not be happy while listening to a radical feminist tirade about the worse qualities of men and how this makes them inferior. To this end, Barbarians hate being degraded as barbarians, and, in some small way, want to make the folk tales of them being demons end. They want to join the civilized world, and they think that only if they conquer enough civilizations, then they'll become civilized.
Barbarians have a distinct culture. Often, their religion is a violent one, where the gods demand human sacrifices. Additionally, laws are most often written by the sword.