From Wikipedia:
The game projects two images, the correct one on the top screen, and an error filled one on the bottom screen. Comparing the two images, the player has to locate and find the inaccuracies or changes from the top screen to the touch screen.
This game is based on the traditional game spot the difference. GamePro.com described the game "like a mix between Where's Waldo and the Hidden Pictures section in Highlights magazine".[1]
The players use none of the face buttons or D-Pad on the DS. The entire game is played using the touch screen and stylus. The player draws a circle using the stylus when the player spots the difference between the top image on the bottom screen. In some instances, the built in microphone is used.