Troubled by the divisive and unfair Tom Robinson trial and trying to create meaning based on what information he has, Jem tells Scout that there are four kinds of people in the "world." He bases the "world," as all people do, on what he knows, which in his case is Maycomb society. What he tells Scout is an accurate summary of how the Maycomb social hierarchy is organized in the 1930s.
He normalizes his own group, the people at the pinnacle of Maycomb's society: white and middle class. He calls this group their "neighbors" and the "ordinary kind," as if they are the model of what humanity should be. Following that, he lists the poor but respectable white people who work hard and don't take charity. These, to him, are the people like the Cunninghams. Below them are the white trash like the Ewells, who don't work hard, are poor, and accept charity. Below them, in Jem's eyes, are the Black people.
Jem's classifications are based on money and race. People who are white and have the money to live in single-family houses near the center of town are at the top, while Black people, who Lee shows are deliberately kept poor so that the middle-class white people can exploit their labor, are on the bottom.
Scout tries to find the shared humanity between all these groups, a humanity which does exist but which is corrupted by the social hierarchy.