Simply put, social prejudice is a preceived notion or judgment that is formed based solely on a person's social standing. Stereotyping or discrimination of a person based on his social standing happen frequently and are definite forms of social prejudice.
In Harper Lee's To Kill a Mockingbird, the theme of prejudice is addressed numerous times; social prejudice is one of the forms of discrimination included in Lee's novel. One example of social prejudice is the town's view of the Ewell family; while Bob Ewell seems to deserve the poor view most people have of him, Mayella Ewell suffers from her position in Maycomb society. Mayella even attempts to improve her environment by planting geraniums, but she is unable to overcome the fact that she is born to a family of very low social standing (and other factors).
A second episode of social prejudice is found in Aunt Alexandra's condescending view of Walter Cunningham, Jr. Aunt Alexandra instructs Scout not to invite Walter to their home because
"...you can scrub Walter Cunningham till he shines, you can put him in shoes and a new suit, but he'll never be like Jem. Besides, there's a drinking streak in that family a mile wide. Finch women aren't interested in that sort of people."
When Scout insists on a reason for her aunt's instructions, Alexander elaborates.
"Because--he--is--trash, that's why you can't play with him. I'll not have you around him, picking up his habits and learning Lord-knows-what..."
Although Aunt Alexandra has no reason to have formed a negative opinion of Walter, his social standing dictates her view of him.
A third example of social discrimination could be found in Aunt Alexandra's opinion of the Finch family's standing being more important and refined than that of most of the other citizens of Maycomb. She believes that the Finch family's history causes it to be more deserving of respect than others.