Aunt Alexandra is certainly one of the biggest snobs of Harper Lee's characters in To Kill a Mockingbird.
By definition a snob is "one who blatantly imitates, fawningly admires, or vulgarly seeks association with those regarded as social superiors; one who tends to rebuff, avoid, or ignore those regarded as inferior; one who has an offensive air of superiority in matters of knowledge or taste." Snobbish behavior, or snobbery, is the act of being a snob.
Alexandra exhibits these traits several times during the novel. She holds her family heritage in much too high regard, considering the Finches one of Maycomb's elite. She feels Walter Cunningham Jr. is "trash" and beneath associating with Scout or visiting their home. Scout combats her aunt's tendencies by purposely asking about questionable members of the Finch family--Cousin Joshua, for example, who was "locked up" for trying to shoot the university president--knowing it will anger her. In Chapter 23, however, Jem and Scout decide not to argue with Alexandra when she berates Walter Jr. Jem comforts the sobbing Scout, soothes her with a Tootsie Roll and then explains that the "Old Family stuff's foolishness" is all that their aunt has to hang on to since "all we've got's background and not a dime to our names."