The game, FreeRice, tests the vocabulary of participants. For each click on a correct answer, the website donates money to buy 10 grains of rice.
Companies advertising on the website provide the money to the WFP to buy and distribute the rice.
http://www.freerice.com/index.php
Although the cynic in me wonder why the companies that sponsor this; Macey's, American Express, Mac, Toshiba, Radisson....don't just donate a few billion $s of their profit instead.