The Nunchuk is an attachment for the Wii Remote. It is plugged into the Wii Remote's expansion port, where the two are connected via a cord.
The Nunchuk contains an analog stick and two buttons on the top of the unit, as well as motion sensing devices inside. The rectangular button is known as the Z button, and the small, circular button is known as the C button. The motion sensing devices are not as tactile and sophisticated as the Wii Remote's.
The Nunchuk is generally held in the left hand, although it can be used in the right hand, a practice recommended in certain games. One Nunchuk is included with every Wii console.
Additional Nunchuk attachments are sold separately for a Manufacturer's Suggested Retail Price of $19.99 USD.
The Nunchuk can be seen a few times in Lady Gaga's music video "Bad Romance".