This is a the game's idea:
Labeling an image means associating word descriptions to it, as shown below. Computer programs can't yet determine the contents of arbitrary images, but the ESP game provides a novel method of labeling them: players get to have fun as they help us determine their contents. If the ESP game is played as much as other popular online games, we estimate that all the images on the Web can be labeled in a matter of weeks!
Having proper labels associated to each image on the Internet would allow for very accurate image search, would improve the accessibility of the Web (by providing word descriptions of all images to visually impaired individuals), and would help users block inappropriate (e.g., pornographic) images from their computers.
It's actually quite fun.
http://www.espgame.org