What the Nintendo nunchuk looks like.
The Nunchuck is the controller that comes with the Wii and is used for 3D navigation games (most of the time), such as Super Mario Galaxy. The nunchuck has 2 buttons, a joystick, and a cable to plug into yourWii Remote; the two buttons are the Z button and the C button. The Nunchuck happens to have sensors integrated inside the controller. The main use of a Nunchuck is to play Mario Kart: Double Dash!! on a Wii without having to use a Nintendo GameCube (controller). It can also be used to play Mario Kart Wii.
External links
An article on the Wii Nunchuck: [1]