A simile is a figure of speech in which you compare two different things using the words "as," or "like." Good examples would be "as strong as an ox," or "like a bull in a china shop." Scout comes up with many wonderful similes in To Kill a Mockingbird. One of my own personal favorites is the colorful way she describes the ladies of Maycomb:
[The ladies] were like soft teacakes with frostings of sweat and sweet talcum.
This particular simile says a lot about the strenuous efforts of Maycomb's female population to appear ladylike and refined in the midst of the intense summer heat.
Then there's Scout's vivid description of Mrs. Dubose, the mean old lady who always yells at Scout and Jem each time they walk past her house. There's something appropriately cold and hard about the simile that Scout uses to describe the old woman, which perfectly captures her icy demeanor:
She was horrible. Her face was the color of a dirty pillowcase, and the corner of her mouth glistened with wet, which inched like a glacier down the deep grooves enclosing her chin.