The most prominent symbols in "The Lottery" are: the black box, the stool, the slips of paper, the stones, and most importantly, the lottery itself.
The lottery in Jackson's story becomes an ironic symbol, because of the reader's anticipation and background knowledge of how lotteries work. Typically, lotteries feature a random game of chance, some type of drawing, in which the winner whose name or slip is drawn wins a prize. In today's modern society with mega-million lotteries, the lottery has come to represent a life-changing event, as people dream of what they would do with their millions if they won the lottery.
In Jackson's story, however, the positive anticipated outcome never arrives; Tessie Hutchinson does not win a fabulous prize. Instead, her fellow villagers pragmatically stone her to death. Jackson uses the lottery as a symbol in her novel to represent the dangers of following tradition merely for tradition's sake, and all of the other symbols incorporated into the story, the lottery paraphanalia, support this larger theme of customs and traditions.