The Nunchuk is one of the few accessories that can be added to the Wii Remote. It can be plugged into the receiver near the end and held in the left (or right, whichever hand) hand for a different way of controlling your character.
In Mario Kart Wii, the Nunckuk is just another way to play the game, but so you don't have to hold the remote sideways.
About half the size of the Wii Remote, the Nunchuk is compact and grooved to fit perfectly in one's hand. The top of the accessory is smooth except for the main part- the control stick (steering in Mario Kart Wii). On the front panel, a small, square button, Z, can be pressed for Item usage, and below that is the circular C button, which allows you to use your rear-view mirror and spot those guys who like to Draft and throw Green Shells. A cord protrudes from the bottom for about 2 1/2 feet and ends in a small chip to plug into the Wii Remote...
The standard Nunchuk was white and was packaged with a Wii, but they could also be bought separately for $20. A lot of other colors were manufactured too, including black (with the black Wii system), red, pink, dark blue, and others.
Now, only Wii U Nunchuks can be purchased, but they work with the Wii as well, with a slightly different shape.