There are several times in The Kite Runner when Khaled Hosseini discusses the Hazaras, and most of them are in connection with the most noble characters in the novel: Hassan and his father Ali. Unfortunately, any description of the Hazara must also include a recounting of the abuse they have suffered for centuries at the hands of the Pashtuns.
Hassan and Ali are Hazaras, the ethnic people in Afghanistan who are the lower and more despised class. They are easily recognizable because they look differently than the ruling class, the Pashtuns. Amir describes Hassan this way:
They called him "flat-nosed" because of Ali and Hassan's characteristic Hazara Mongoloid features. For years, that was all I knew about the Hazaras, that they were Mogul descendants, and that they looked a little like Chinese people.
The Hazaras are of the Shi'a religion, and in the novel Amir knows little about them until he reads a book from his father's library, since they are barely even mentioned in school textbooks. Amir is stunned to find an entire chapter given to the Hazaras, something he had barely ever even given a thought to, despite his growing up with Hassan. Without saying it, Amir is admitting that the Hazaras are not worthy of any attention or study. He reads that
the Pashtuns, had persecuted and oppressed the Hazaras. It said the Hazaras had tried to rise against the Pashtuns in the nineteenth century, but the Pashtuns had "quelled them with unspeakable violence." The book said that my people had killed the Hazaras, driven them from their lands, burned their homes, and sold their women. The book said part of the reason Pashtuns had oppressed the Hazaras was that Pashtuns were Sunni Muslims, while Hazaras were Shi'a. The book said a lot of things I didn't know.... It also said some things I did know, like that people called Hazaras mice-eating, flat-nosed, load-carrying donkeys.
The fact that the Hazara look so different from the Pashtuns makes them an easy target for ridicule and worse, as in the case of Assef's assault of Hassan.
One other description (both physical and social) comes from Amir as an adult looking back on his relationship with Hassan, a Hazara:
The curious thing was, I never thought of Hassan and me as friends either. Not in the usual sense, anyhow. Never mind that we taught each other to ride a bicycle with no hands, or to build a fully functional homemade camera out of a cardboard box. Never mind that we spent entire winters flying kites, running kites. Never mind that to me, the face of Afghanistan is that of a boy with a thin-boned frame, a shaved head, and low-set ears, a boy with a Chinese doll face perpetually lit by a harelipped smile.
Never mind any of those things. Because history isn't easy to overcome. Neither is religion. In the end, I was a Pashtun and he was a Hazara, I was Sunni and he was Shi'a, and nothing was ever going to change that. Nothing.
This is a vivid reminder that what the Hazara look like is one thing that separates the Hazara, from the Pashtuns, but the divide goes much deeper than that. A history of racial and religious division is not something which can be easily surmounted in a society or in a friendship. Even at the end of the novel, when Amir has come to regret his treatment of Hassan, the divide is still almost insurmountable.