Jackson defers the revelation of the lottery's true purpose until the end of the story, when "the winner," Tess Hutchison, is stoned to death by friends and family. This shocking event marks a dramatic turning point in how we understand the story. We think that Jackson uses stoning as a metaphor for the innate savagery that can lurk beneath a modern, civilized façade. See "Symbols, Imagery, Allegory" for more on this analogy.
The ending is a turning point in other ways as well. One critic notes that the ending transforms "The Lottery" from realism to symbolism, as we suddenly understand the town and its inhabitants as being symbolic rather than actual. For Tess Hutchinson, the ending of the lottery is certainly not what she expects. Although she began the story as an eager latecomer to the event, the story's conclusion brings out her hypocrisy: Tess Hutchinson is quite willing to participate in group-sponsored violence until she becomes its victim.