You'll find at least at least two answers to very similar questions at enotes. See the links provided below.
I want to add a detal that doesn't seem to discussed in those answers. The novel has multiple expressions of a caste system. Aunt Alexandra discusses her version in chapter 13, for example, and Jem expresses his own version in chapter 23. Does Scout also have her own version? (Right now, all I can remember from her is an anti-caste statement: "I think there's just one kind of folks. Folks.") It may be interesting to compare the different versions, looking at what each version emphasizes and to what extent they agree or disagree.
Caste systems are inherently flawed, according to the modern democratic sentiments that readers of Lee's novel are likely to have, because these systems limit the potential of any one individual. People are born into a caste (a fixed place in a social hierarchy) that often determines just about everything they will do in their entire life -- where they will live, who they might marry, what sort of job they might have, and so on. Caste systems are often (nearly always?) based on family lines, and thus visible racial and ethnic differences are often used as markers of caste.