Society as it is portrayed in the novel is shown to be riddled with prejudice. Racism is only one form of this. Blacks and whites are strictly segregated in the town of Maycomb, and blacks are generally looked down upon by the whites. They generally only appear in white society in the capacity of servant, like the Finch's housekeeper Calpurnia or Zeebo the garbage collector. The most dramatic instance of racism in the novel is, of course, the trial of Tom Robinson, unfairly accused and convicted of rape, even when all the evidence suggests otherwise. Tom doesn't stand a chance simply because of his colour.
Prejudice is also seen to be entrenched in Maycomb in other ways. People of different social class and family background are not expected to mix. This kind of prejudice is most notable in characters such as Aunt Alexandra who always insists on the superiority of the Finch family background and disapproves of Scout and Jem mixing with the lower orders. As Scout and Jem grow up they come to realise just how strong prejudices can be in society, keeping people apart and causing all kinds of problems. The novel shows how they lose their childlike innocence as they come to experience more and more of the world and its ways, but they also gain in understanding.